Ccent notes part 3

CCENT Notes

Part 3 – IP Routing

Ref : CCENT/CCNA ICND1 Official Exam Certification Guide, Second Edition
by Wendell Odom

It is highly recommended that you read at least once the above study guide to make full
use of this notes, it is expected that there may be minor errors in this notes, please
always refer the study guide for accurate information.

(Jojo Jacob - CCENT)

TABLE OF CONTENTS

Chapter 12 – IP Addressing and Subnetting........................................................................3
Chapter 13 – Operating Cisco Routers..............................................................................38
Chapter 14 – Routing Protocol Concepts and Configuration............................................47
Chapter 15 – Troubleshooting IP Routing.........................................................................58

2

Chapter 12 – IP Addressing and Subnetting

List of all possible valid network numbers….reference table for the number of network,
size of the network part, size of the host part, for Class A,B and C ip networks.
Class A Class B Class C
First Octect range 1 to 126 128 - 191 192 – 223
Valid Network Numbers 1.0.0.0 to 128.0.0.0 192.0.0.0 to
126.0.0.0 191.255.0.0 223.255.255.0
Number of networks in this Class 2^7–2 2 ^ 14 = 2 ^ 21=
= 128 16,384 2,097,152
Number of hosts per network 2 ^ 24 – 2 = 2 ^ 16 – 2 = 2^8–2=
16,777,214 65,534 254
Size of network part of the address 1 2 3
(bytes)
Size of hosts part of the address 3 2 1
(bytes)
^ raised to
CLASS A (7 Network Bits)
0 N N N N N N N H H H H H H H H H H H H H H H H H H H H H H H H
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

CLASS B (6 + 8 = 14 Network Bits)
1 0 N N N N N N H H H H H H H H H H H H H H H H H H H H H H H H
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

CLASS C (5 + 16 = 21 Network Bits)
1 1 0 N N N N N H H H H H H H H H H H H H H H H H H H H H H H H
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Class A , B and C network, network and host parts and default mask
Class of Size of network part of Size of host part of Default mask for
Address address in bits address in bits each class of
network
A 8 24 255.0.0.0.
B 16 16 255.255.0.0
C 24 8 255.255.255.0

3

RFC 1918 Private IP Address Space
Private IP Networks Class of Networks Number of Networks
10.0.0.0 through 10.0.0.0 A 1
172.16.0.0 through 172.31.0.0 B 16
192.168.0.0 through 192.168.255.0 C 255

IPv6 – 128 bit IP address : can provide 10 ^ 38 IP Addresses.
Currently in the earth there are 10 ^ 10 people living.

IPv4 vs IPv6
Feature IPv4 IPv6
Size of address 32 bits , 4 octects 128 bits , 16 octects
Example address 10.1.1.1 0000:0000:0000:0000:FFFF:FFFF:0A01:0101
Same address ---- ::FFFF:FFFF:0A01:0101
abrreviated
Number of possible 2 ^ 32 2 ^ 128 , or roughly 3.4 x 10 ^ 38
addresses, ignoring Approx 4 billion
reseved addresses

Bitwise Boolean AND example
Decimal Binary
Address 150.150.2.1 10010110 10010110 00000010 00000001
Mask 255.255.255.0 11111111 11111111 11111111 00000000
Result of AND 150.150.2.0 10010110 10010110 00000010 00000000

255.255.255.0 – 11111111 11111111 11111111 00000000
Is written as a /24 subnet mask.

Binary process to convert between Dotted decimal and prefix notation

Step 1. Covert the dotted decimal mask to binary
Step 2. Count the number of binary 1s in the 32 bit binary mask, this is the value of the
prefix notation mask

Eg. 255.255.240.0 converts to….
11111111 11111111 11110000 00000000
The mask has 20 binary 1s, so the prefix notation of the same mask is /20.

Convert prefix notation to a dotted decimal format…

Step 1. write down x binary 1s, where x is the value listed in the prefix notation of the
mask

4

Step 2. Write down binary 0s after binary 1s until you have written down all the 32 bits

Step 3. Convert this binary number, 8 bit at a time to dotted decimal format.

Eg. /20 converts to
11111111 11111111 11110000 00000000
255.255.240.0

Nine possible Decimal Numbers in a subnet mask
Subnet mask’s Binary Equivalent Number of Binary Number of Binary
Decimal Octect 1s 0s
0 00000000 0 8
128 10000000 1 7
192 11000000 2 6
224 11100000 3 5
240 11110000 4 4
248 11111000 5 3
252 11111100 6 2
254 11111110 7 1
255 11111111 8 0

Convert a dotted decimal subnet mask format to a prefix format….

Step 1. Start with a prefix value of 0
Step 2. For each dotted decimal octect, add the number of binary 1s, listed for that
decimal value in the table above
Step 3. Prefix length is /x , where x is the sum calculated in Step 2.

Eg. 255.255.240.0

Start from 0
First octect 255 add 8
Second octect 255 add 8
Third octect 240 add 4
Fourth octect 0 add 0

Prefix is 20

Converting a /20 subnet into dotted decimal format….

Step 1. Divide x by 8 ( x / 8) , noting the number of times 8 goes fully into x , the
dividend represented as d, and the number left over, the reminder represented as r.

Step 2. write down d octets of 255,

5

Step 3. for the next octet, find the decimal number that begins with r, binary 1s,
Followed by all binary 0s.

Step 4. for any remaining octets write down value 0

20 / 8 = 2 and a reminder of 4

2 octects with all binary 1s , third octed with 1 birary 1 and 4 binary 0 , and last one with
all binary 0s.

255.255.240.0

Appendix D. Problem Set 1.

255.240.0.0 - 8+4+0+0+ = /12
255.255.192.0 – 8+8+2+0 = /18
255.255.255.224 – 8+8+8+3 = /27
255.254.0.0 – 8+7+0+0 = /15
255.255.248.0 - 8+8+5+0 = /21

/30 - 255.255.255.252
/25 - 255.255.255.128
/11 - 255.224.0.0
/22 - 255.255.252.0
/24 - 255.255.255.0

255.240.0.0 - 11111111 11110000 00000000 00000000 - /12
255.255.192.0 – 11111111 11111111 11000000 00000000 /18
255.255.255.224 – 11111111 11111111 11111111 11100000 /27
255.254.0.0 – 11111111 11111110 00000000 00000000 /15
255.255.248.0 - 11111111 11111111 11111000 00000000 /21

/30 - 11111111 11111111 11111111 11111100 255.255.255.252
/25 - 11111111 11111111 11111111 10000000 255.255.255.128
/11 - 11111111 11100000 00000000 00000000 255.224.0.0
/22 - 11111111 11111111 11111100 00000000 255.255.252.0
/24 - 11111111 11111111 11111111 00000000 255.255.255.0

6

ANALYSING AND CHOOSING SUBNET MASKS

SLSM – Static Length Subnet Mask
VLSM – Variable Length Subnet Mask

How to find the size of the Network , Subnet and Host part of an IP Address…

• The network part of the address is always defined by the class rule
• The host part of the address is defined by the subnet mask, the number of binary zeros
in the subnet mask defines the number of host bits.
• Subnet part of the address is what is left over in the 32 bit address

IP Address : 8.1.4.5
Mask : 255.255.0.0
Class : A
Network Bits : 8
Host bits : 16
Subnet bits : 32 – 24 = 8

IP Address : 130.4.102.1
Mask : 255.255.255.0
Class : B
Network Bits : 16
Host bits : 8

IP Address : 199.1.1.100
Mask : 255.255.255.0
Class : C
Network Bits : 24
Host bits : 8

Facts about how the subnet mask identifies part of the structure of an IP address

• A subnet masks binary 1s define the combined network and subnet part of an IP
address
• The masks binary 0s define the hosts part of the IP address
• Class rules define the size of the network part

7

Finding the Network, Subnet and Host part using binary…..

Step 1. Compare the first octet of the address to the table of Class A,B or C addresses,
write down the number of network bits depending on the address class.

Step 2. Find the number of hosts bits by

a. Converting subnet masks to binary
b. Counting the number of binary 0s in the mask

Step 3. Calculate the number of subnet bits by subtracting the combined network and
hosts bits from 32.

Decimal Process : Finding the Network, Subnet and Hosts bits in a subnet mask

Step 1. Compare the first octet of the address to the table of Class A,B or C addresses;
write down the number of network bits based on the address class.

Step 2. If the mask is in dotted decimal format, convert the mask to prefix format

Step 3. To find the number of host bits, subtract the prefix length from 32

Step 4. Calculate the number of subnet bits by subtracting the combined network and
hosts bits from 32

Given a class full network number and a single subnet mask is used throughout the
classfull network.

Number of subnets :- 2 ^ s , where s is the number of subnet bits
Number of hosts :- 2 ^ h , where h is the number of host bits

When to use which formula for number of subnets
Use the 2 ^ s – 2 formula, and avoid using Use the 2 ^ s formula, and use the zero and
the zero subnet and broadcast subnet if… broadcast subnet if…
Classful routing protocol Classless routing protocol
RIP version 1 or IGRP as the routing RIP Version 2, EIGRP or OSPF as the
protocol routing protocol
The no ip subnet zero command is The ip subnet zero command is configured
configured or ommited (default)
VLSM is used
No other clues provided

8

Analysing the subnet mask

8.1.4.5 /16

Hosts bits : 32 – 16 = 16
Class A network bits : 8
Number of subnets = 2 ^ 8 = 256
Number of host = 2 ^ 16 – 2 = 65,534

130.4.102.1/24

Hosts bits : 32 – 24 = 8
Class B network bits : 16
Number of host = 2 ^ 8 – 2 = 254

199.1.1.100/24

Hosts bits : 32 – 24 = 8
Class C network bits : 24
Number of host = 2 ^ 8 – 2 = 254

130.4.102.1/22

Hosts bits : 32 – 22 = 10
Class B network bits : 16
Number of host = 2 ^ 8 – 2 = 1022

9

199.1.1.100/27

Hosts bits : 32 – 27 = 5
Class C network bits : 24
Number of host = 2 ^ 5 – 2 = 30

Number of bits in the host Maximum number of hosts Maximum number of
or subnet field (2 ^ h – 2) subnets (2 ^ s )
1 0 2
2 2 4
3 6 8
4 14 16
5 30 32
6 62 64
7 126 128
8 254 256
9 510 512
10 1022 1024
11 2046 2048
12 4094 4096
13 8190 8192
14 16,382 16,384

Finding the only possible MASK

With a Class B network 130.1.0.0 , what is the only subnet mask you can use to have
upto 200 subnets and 200 hosts per subnets???

To have 200 subnets you need to have atleast 8 subnet bits
( 2 ^ 8 = 256) , (2 ^ 7 is only 128)

Only possible subnet so is…(16 bits for network part as it is a Class B network).

NNNNNNNN NNNNNNNN SSSSSSSS HHHHHHHH

11111111 11111111 11111111 00000000

255.255.255.0 or a /24 mask.

10

Finding the multiple possible MASKs

With a Class B network, what are the subnet masks you can use to have upto 50 subnets
and 200 hosts in the largest subnet???

To have 50 subnets you need to have atleast 6 subnet bits
( 2 ^ 6 = 64) , (2 ^ 5 is only 32)

To have 200 hosts you need to have atleast 8 host bits
( 2 ^ 8 - 2 = 254) , (2 ^ 7 is only 128)

Possible subnet format is …(16 bits for network part as it is a Class B network).

NNNNNNNN NNNNNNNN SSSSSSXX HHHHHHHH

XX – can be either subnet or host bits

That mean you may get 2 ^ 2 = 4 possible combination of subnet masks, however

All masks must start with one unbroken consecutive string of binary 1s, followed by one
unbroken consecutive string of binary 0s.

So the possible mask are listed below but only three are valid.

11111111 11111111 11111111 00000000 (8 subnets , 8 hosts)
11111111 11111111 11111110 00000000 (7 subnets, 9 hosts)
11111111 11111111 11111100 00000000 (6 subnets, 10 hosts)
11111111 11111111 11111101 00000000 (INVALID MASK)

255.255.255.0 prefix format /24
255.255.254.0 prefix format /23
255.255.252.0 prefix format /22

Choosing the subnet that maximizes the number of subnets or hosts

The mask with the most subnet bits : the mask with the wildcard bits set to binary 1s,
there by increasing the subnet bits, maximizes the number of subnets and minimizes the
number of hosts per subnet.

The mask with most host bits : the mask with the wildcard bits set to binrary 0s, thereby
making the host part of the subnet address larger, maximizes the number of hosts per
subnet and minimizes the number of subnets.

11

Choosing a subnet mask steps …..

Step 1. Find the number of network bits (N) based on the Class A,B or C rules
Step 2 : Find the number of subnet bits (S) based on the formula 2 ^ s , such that 2 ^ s >
or = the number of required subnets.
Step 3. Find the number of host bits (H) based on the formula 2 ^ h – 2, such that 2 ^ h –
2 is >= the number of required hosts.
Step 4. Write down starting from the left N+S binary 1s
Step 5. Write down starting from right H binary 0s
Step 6. If the binary 1s and 0s together adds up to less than 32
a. fill in the remaining ‘wildcard’ bit positions with X between binary 1s and 0s.
b. find all combination of bits for the wildcard bit positions, meeting the requirement of
having one unbroken consecutive string of binary 1s on left.
Step 7. convert the mask to decimal or prefix format
Step 8. To find the mask that maximizes the number of subnets pick the mask with most
binary 1s init, and to find the mask that maximizes the number of hosts pick the mask
with most binary 0s.

Problem set 2. Analysing unsubnetted IP addresses

10.55.44.3

Class of the address : A
Number of octets in the network part : 1
Number of octets in the host part : 3
Network number : 10.0.0.0
Network broadcast number : 10.255.255.255

128.77.6.7

Class of the address : B

192.168.76.54

Class of the address : C

12

190.190.190.190

Class of the address : B

9.1.1.1

Class of the address : A

200.1.1.1

Class of the address : C

Problem Set 3. Interpreting existing subnet masks

10.66.5.99 255.255.254.0

Steps
Mask in prefix format /23
Class : A

Results
Network bits 8
Subnet bits 32 – (8+9) = 15
Host bits 9
Number of subnets in the network 2 ^ 15 = 32,768
Number of hosts per subnet 2 ^ 9 – 2 = 510

13

172.16.203.42 255.255.252.0

Steps
Class : B

Results
Network bits 16
Subnet bits 32 – (16+10) = 6
Host bits 10
Number of subnets in the network 2 ^ 6 = 64

192.168.55.55 255.255.255.224

Steps
Class : C

Results
Network bits 24
Subnet bits 32 – (24+5) = 3
Host bits 5

10.22.55.87 /30

Steps
Class : A

Results
Network bits 8
Subnet bits 32 – (8+2) = 22
Host bits 2
Number of subnets in the network 2 ^ 22 = 4,194,304

172.30.40.166 /26

14

Steps
Class : B

Results
Network bits 16
Subnet bits 32 – (16+6) = 10
Host bits 6

192.168.203.18 /29

Steps
Class : C

Results
Network bits 24
Subnet bits 32 – (24+3) = 5
Host bits 3

Problem set 4 : Choosing Subnet Masks

Network 10.0.0.0 needs 50 subnets and 200 hosts/subnets

Class : A
Network bits : 8
Min. Subnet bits 6
Min. Host bits 8

NNNNNNNN SSSSSSXX XXXXXXXX HHHHHHHH

10.0.0.0 /14 (Maximum number of hosts)
10.0.0.0 /15
10.0.0.0 /16
10.0.0.0 /17
10.0.0.0 /18

15

10.0.0.0 /19
10.0.0.0 /20
10.0.0.0 /21
10.0.0.0 /22
10.0.0.0 /23
10.0.0.0 /24 (Maximum number of subnets)

Network 172.32.0.0 need 125 subnets and need 125 hosts/subnet

Class : B
Network bits : 16
Min. Subnet bits : 7
Min. Host bits : 7

NNNNNNNN NNNNNNNN SSSSSSSX XHHHHHHH
172.32.0.0 /23 255.255.254.0 (Maximum number of hosts)
172.32.0.0 /24 255.255.255.0
172.32.0.0 /25 255.255.255.128 (Maximum number of subnets)

Network 192.168.44.0 need 15 subnets and 6 host/subnet

Class : C
Network bits 24
Min. subnet bits : 4
Min. host bits : 3

NNNNNNNN NNNNNNNN NNNNNNNN SSSSXHHH

192.168.44.0 /28 255.255.255.240 (Maximum number of hosts/subnet)
192.168.44.0 /29 255.255.255.248 (Maximum number of subnets)

10.0.0.0 300 subnets and 500 hosts/subnet

Class : A
Network bits : 8
Min. host bits : 9

NNNNNNNN SSSSSSSS SXXXXXXH HHHHHHHH

10.0.0.0 /17 255.255.128.0 Maximum hosts/subnets

16

10.0.0.0 /18 255.255.192.0
10.0.0.0 /19 255.255.224.0
10.0.0.0 /20 255.255.240.0
10.0.0.0 /21 255.255.248.0
10.0.0.0 /22 255.255.252.0
10.0.0.0 /23 255.255.254.0 Maximum subnets

172.32.0.0 500 subnets and 15 hosts/subnet

Class : B
Network bits : 16
Min. subnets bits : 9
Min. host bits : 5

NNNNNNNN NNNNNNNN SSSSSSSS SXXHHHHH
172.32.0.0 / 25 255.255.255.128 Maximum hosts/subnet
172.32.0.0 / 26 255.255.255.192
172.32.0.0 / 27 255.255.255.224 Maximum subnets

Network 172.16.0.0 2000 subnets 2 hosts/subnet

Class : B
Network hosts : 16
Min. host bits : 2

NNNNNNNN NNNNNNNN SSSSSSSS SSSXXXHH

172.16.0.0 /27 255.255.255.224 Maximum number of hots/subnet
172.16.0.0 /28 255.255.255.240
172.16.0.0 /29 255.255.255.248
172.16.0.0 /30 255.255.255.252 Maximum number of subnets

ANALYSING EXISTING SUBNETS : BINARY

Learning resident subnet number using Boolean AND operation….

Step 1. Convert the IP address from decimal to binary
Step 2. Convert the subnet mask to binary and write it down under the binary ip address
Step 3. Perform a bit wise Boolean AND operation of the two numbers…
Step 4. Convert the resulting binary number back to decimal, to get the subnet number

17

Address 8.1.4.5 00001000 00000001 00000100 00000101
Mask 255.255.0.0 11111111 11111111 00000000 00000000
Subnet Number 8.1.0.0 00001000 00000001 00000000 00000000

Address 130.4.102.1 10000010 00000100 01100110 00000001
Mask 255.255.255.0 11111111 11111111 11111111 00000000
Subnet Number 130.4.102.0 10000010 00000100 01100110 00000000

Address 199.1.1.100 11000111 00000001 00000001 01100100
Mask 255.255.255.0 11111111 11111111 11111111 00000000
Sub. Number 199.1.1.0 11000111 00000001 00000001 00000000

Address 130.4.102.1 10000010 00000100 01100110 00000001
Mask 255.255.252.0 11111111 11111111 11111100 00000000
Subnet
Number 130.4.100.0 10000010 00000100 01100100 00000000

Address 199.1.1.100 11000111 00000001 00000001 01100100
Mask 255.255.255.224 11111111 11111111 11111111 11100000
Subnet
Number 199.1.1.96 11000111 00000001 00000001 01100000

Binary Short cut….

• Record decimal mask in the first row of the table, and decimal IP address below it
• For any mask octect of value 255, copy the IP address’s octet value for the same octet
of the decimal subnet number
• Similarly for any mask octet value of 0, write down decimal 0 for the same octet of
the subnet number
• If the subnet number has still has one remaining octet to be filled in, then
o Convert the remaining octet of the ip address to binary
o Convert the remaining octet of the mask to binary
o AND the two 8-bit numbers together
o Covert the 8 bit number to decimal, and place the value in the remaining octet
of the subnet number

Address 199.1.1.100
Mask 255.255.255.0
Sub. Number 199.1.1.0

Address 130.4.102.1 01100110
Mask 255.255.252.0 11111100
Subnet
Number 130.4.100.0 01100100

18

Calculating the Broadcast address – Binary

Address 8.1.4.5
Mask 255.255.0.0 nnnnnnnn nnnnnnnn hhhhhhhhh hhhhhhh
Subnet
Address 8.1.0.0 00001000 00000001 00000000 00000000
Broadcast
Address 8.1.255.255 00001000 00000001 11111111 11111111

Address 130.4.102.1
Mask 255.255.255.0
Subnet
Address 130.4.102.0
Broadcast
Address 130.4.102.255
Address 199.1.1.100
Mask 255.255.255.0
Subnet
Address 199.1.1.0
Broadcast
Address 199.1.1.255

Address 130.4.102.1 01100110
Mask 255.255.252.0 11111100
Subnet
Address 130.4.100.0 01100100
Broadcast
Address 130.4.103.255 01100111

Address 199.1.1.100 0110 0100
Mask 255.255.255.224 1110 0000
Subnet
Address 199.1.1.96 0110 0000
Broadcast
Address 199.1.1.127 0111 1111

Steps to determine Subnet Broadcast Address…..

• Step 1. Write down the subnet number (IP address), and subnet mask in binary form,
• Step 2. Separate the network/subnet and host bits by a vertical line
• Step 3. To find the subnet broadcast address in binary

19

o Copy the bits of the subnet number (IP address) that are to the left of the
vertical line
o Write down binary 1s for the (HOST) bits to the right of the vertical line

• Step 4. Convert the 32 bit binary subnet broadcast address to decimal, 8 bits at a time,
ignoring the vertical line

Steps to find the first and last IP address in a subnet

Step 1. To find the first IP address, Copy the subnet number but add 1 to the fourth octet
Step 2. To find the last IP address, Copy the subnet broadcast address but substract1 from
the fourth octet

Address 8.1.4.5
Mask 255.255.0.0 nnnnnnnn nnnnnnnn hhhhhhhhh hhhhhhh
Subnet
Address 8.1.0.0 00001000 00000001 00000000 00000000
Broadcast
Address 8.1.255.255 00001000 00000001 11111111 11111111
First
Address 8.1.0.1
Last
Address 8.1.255.254

Address 130.4.102.1
Mask 255.255.255.0
Subnet
Address 130.4.102.0
Broadcast
Address 130.4.102.255
First
Address 130.4.102.1
Last
Address 130.4.102.254

Address 199.1.1.100
Mask 255.255.255.0
Subnet
Address 199.1.1.0
Broadcast

20

Address 199.1.1.255
First
Address 199.1.1.1
Last
Address 199.1.1.254

Address 130.4.102.1 01100110
Mask 255.255.252.0 11111100
Subnet
Address 130.4.100.0 01100100
Broadcast
Address 130.4.103.255 01100111
First
Address 130.4.100.1
Last
Address 130.4.103.254

Address 199.1.1.100 0110 0100
Mask 255.255.255.224 1110 0000
Subnet
Address 199.1.1.96 0110 0000
Broadcast
Address 199.1.1.127 0111 1111
First
Address 199.1.1.97
Last
Address 199.1.1.126

Finding subnet address, broadcast address and range of address using Decimal
– Difficult Mask
Subnet Chart 130.4.102.1 / 255.255.252.0
Octet 1 2 3 4 Comments
Mask 255 255 252 0
Address 130. 4 102 1
Subnet Address 130 4 100 0 Magic number = 256 – 252 = 4
100 is the multiple of 4 closes to
but not higher than 102
First Address 130 4 100 1 Add 1 to the subnets last octet
Last Address 130 4 103 25 Subtract 1 from broadcast
4 address’s fourth octet
Broadcast Address 130 4 103 25 Subnet’s interesting octet +
5 magic number – 1 (100+4 – 1)

21

Summary of decimal process to find the subnet, broadcast and range

Step 1. Write down the subnet mask in the first empty row of the subnet chart, and the IP
address in the second empty row.

Step 2. Find the octet for which subnet mask’s value is not 255 or 0. This octet is called
the interesting octet. Draw a dark rectangle around the interesting octet’s column of the
table, top to bottom.

Step 3. Record the subnet numbers value for the uninteresting octets as follows

a. for each octet to the left of the rectangle, drawn in step 2, copy the IP address value in
the same octet.

b. for each octet to the right of the rectangle: write down decimal 0

Step 4. To find the subnet numbers value for this interesting octet

a. calculate the magic number by subtracting the subnet mask’s interesting octet value
from 256

b. calculate the multiple of magic number starting from 0 through to 256

c. write down the interesting octet value, calculated as follows, Find the multiple of
magic number that is closest to, but not higher than the IP Address’s interesting octet
value.

Step 5 : Find the subnet broadcast address as follows

a. for each subnet mask octet to the left of the rectangle, copy the IP address octet value

b. for each subnet mask octet to the right of the rectangle, write down 255

c. find the value for the interesting octet by adding the subnet number’s value in the
interesting octet to the magic number and subtract 1.

Step 6. To find the first IP address, copy the decimal subnet number, but add 1 to the
fourth octet.

Step 7. To find the last IP address, copy the decimal subnet broadcast address, but
subtract 1 from the fourth octet.

22

Problem 1
10.180.10.18 255.192.0.0

Size of Network Part 8
Size of Subnet Part 2
Size of Host part 22
Number of hosts per subnet 2 ^ 22 – 2 = 4,194,302
Number of subnets 2^2=4

255.192.0.0
10.180.10.18
(256 – 192 ) = 64
64 x 3 = 192
64 x 2 = 128

Subnet Number 10.128.0.0
Broadcast Address 10.191.255.255

Range of valid IP addresses
First Address 10.128.0.1
Last Address 10.191.255.254

Problem 2
10.200.10.18 255.224.0.0


255.224.0.0
10.200.10.18
(256 – 224 ) = 32
32 x 6 = 192



23

Problem 3
10.100.18.18 255.240.0.0

Number of subnets 2 ^ 4 = 16

255.240.0.0
10.100.18.18
(256 – 240 ) = 16
16 x 6 = 96



Problem 4
10.100.18.18 255.248.0.0

Number of hosts per subnet 2 ^ 19 – 2 = 524,286

255.248.0.0
10.100.18.18
(256 – 248 ) = 8
8 * 12 = 96



24

Problem 5
10.150.200.200 255.252.0.0


255.252.0.0
10.150.200.200
(256 – 252 ) = 4
37 * 4 = 148



Problem 6
10.150.200.200 255.254.0.0


255.254.0.0
10.150.200.200
(256 – 254 ) = 2
75 * 2 = 150



25

Problem 7
10.220.100.18 255.255.0.0


255.255.0.0
10.220.100.18
(256 – 255 ) = 1
220 * 1 = 220



Problem 8
10.220.100.18 255.255.128.0


255.255.128.0
10.220.100.18
(256 – 128 ) = 128
0 * 128 = 0



26

Problem 9
172.31.100.100 255.255.192.0


255.255.192.0
172.31.100.100
(256 – 192 ) = 64
1 * 64 = 64



Problem 10
172.31.100.100 255.255.224.0


255.255.224.0
172.31.100.100
(256 – 224 ) = 32
3 * 32 = 96



27

Problem 11
172.31.200.10 255.255.240.0


255.255.240.0
172.31.200.10
(256 – 240 ) = 16
12 * 16 = 192



Problem 12
172.31.200.10 255.255.248.0


255.255.248.0
172.31.200.10
(256 – 248 ) = 8
25 * 8 = 200



28

Problem 13
172.31.50.50 255.255.252.0


255.255.252.0
172.31.50.50
(256 – 252 ) = 4
12 * 4 = 48



Problem 14
172.31.50.50 255.255.254.0

Size of Host part 9

255.255.254.0
172.31.50.50
(256 – 254 ) = 2
25 * 2 = 50



29

Problem 15
172.31.140.14 255.255.255.0

Size of Host part 8

255.255.255.0
172.31.140.14



Problem 16
172.31.140.14 255.255.255.128

Size of Host part 7

255.255.255.128
172.31.140.14
256 – 128 = 128
0 * 128 = 0



30

Problem 17
192.168.15.150 255.255.255.192

Size of Host part 6

255.255.255.192
192.168.15.150
256 – 192 = 64
2 * 64 = 128



Problem 18
192.168.15.150 255.255.255.224

Size of Host part 5

255.255.255.224
192.168.15.150
256 – 224 = 32
4 * 32 = 128



31

Problem 19
192.168.100.100 255.255.255.240

Size of Host part 4

255.255.255.240
192.168.100.100
256 – 240 = 16
6 * 16 = 96


Last Address 192.168.100.110

Problem 20
192.168.100.100 255.255.255.248

Size of Host part 3
Number of hosts per subnet 2^3–2=6

255.255.255.248
192.168.100.100
256 – 248 = 8
12 * 8 = 96


Last Address 192.168.100.102

A NOTE : In the above examples, Number of subnets means, maximum number of
subnets possible for the subnets mask (eg. 255.255.255.248),

32

Subnet address and Broadcast address are the subnet and broadcast address for the
subnet the given IP addresss 192.168.100.100 belongs to. (see the binary version in
appendix d for more details)

Problem 21
192.168.15.230 255.255.255.252

Size of Host part 3
Number of hosts per subnet 2^3–2=6

255.255.255.248
192.168.100.100
256 – 248 = 8
12 * 8 = 96


Last Address 192.168.100.102

Problem 22
10.1.1.1 255.248.0.0


255.248.0.0
10.1.1.1
256 – 248 = 8
0*8=0



33

Problem 23
172.16.1.200 255.255.240.0


255.255.240.0
172.16.1.200
256 – 240 = 16
0 * 16 = 0



Problem 24
172.16.0.200 255.255.255.192

Size of Host part 6

255.255.255.192
172.16.0.200
256 – 192 = 64
3 * 64 = 192



34

Problem 25
10.1.1.1 255.0.0.0




Finding all subnets with fewer than 8 subnet bits

Generic list All Subnet Chart
Octect 1 2 3 4
Mask 255 255 252 0
Magic Number 4
Network number/Zero subnet 130 4 0 0
Next subnet 130 4 4 0
Last subnet 130 4 248 0
Broadcast subnet 130 4 252 0
Out of range (used by process) 130 4 256 0

Step 1. Write down the subnet mask in decimal, in the first empty row of the table
Step 2. Identify the interesting octet, which is the octet with value other than 255 or 0,
and draw a rectangle around the column of the interesting octet.
Step 3. Calculate the magic number by subtracting the mask’s interesting octet from 256
Step 4. Write down the classful network number (zero subnet number)
Step 5. To find each successive subnet number
a. for the three un-interesting octets copy the previous subnet numbers value
b. for the interesting octet add the magic number to the previous subnet numbers
interesting octet value.
Step 6. Once the sum calculated in stp 5.b becomes 256, stop the process, the number
with 256 in it is out of range, and previous subnet number is the broadcast subnet
number.

35

Finding all subnets with exact 8 subnet bits

The subnet octet is the interesting octet, to find all the subnets add 1 to the interesting
octet till it reaches 256.

Finding all subnets with more than 8 subnet bits

The process follows the same five steps as with fewer than 8 subnet bits.

Step 6. When any steps addition results is in sum of 256
a. for the octet whose sum would have been 256 write down 0
b. for the octet to the left add 1 to the previous subnet’s value in that octet
c. for any other octet copy the value of the same octet in the previous subnet number
d. start again with step 5

Step 7. each time the process results in a sum of 256, repeat step 6 of this process

Step 8. Repeat the steps until the addition in step 6b, would actually change the value of
the network portion of the subnet number

Octect 1 2 3 4
Mask 255 255 255 192
Magic Number 64
Network number/Zero subnet 130 4 0 0
First non-zero subnet number 130 4 0 64
Next subnet (add 1 to the third octet, and 130 4 1 0
write 0 in the fourth octet)

Broadcast subnet 130 4 255 192

Definitions….

Bitwise Boolean AND : A Boolean AND between two numbers of the same length where
the first bit in each number is ANDed and the second bit and so on

Boolean AND : A math operation performed on a pair of one digit binary numbers, the
result is another one digit binary number, binary 1 and 1 yields a result of binary 1, all
other combinations yielding binary 0.

36

Broadcast subnet : When subnetting a Class A, B or C network, the one network in each
classful network, for which all subnet bits have a value of binary 1s. The subnet
broadcast address in this subnet has the same numeric value as the classful network’s
network wide broadcast address.

Classful network : An IPv4 Class A,B or C network, called classful network, because
these networks are defined by the class rules for IPv4 addressing.

Default Mask : The mask used in Class A,B or C network, that does not create any
subnets, specifically mask 255.0.0.0 for Class A, 255.255.0.0 for Class B, and
255.255.255.0 for a Class C network.

Prefix notation , CIDR notation : A shorter way to write subnet mask, in which number of
binary 1s in the mask is simply written in decimal. For instance /24 denotes the subnet
mask with 24 binary 1 bits in the subnet mask.

Private IP address : IP addresses within Class A,B and C, networks that are set aside for
use within a private organization. These addresses are defined by RFC 1918, and are not
routable through internet.

Public IP Address : An IP address that is part of a registered network number, as assigned
by an Internet Assigned Numbers Authority (IANA) member agency. Routers in the
internet forward (route) publicly assigned network numbers.

Subnet : Sub division of Class A, B or C network as configured by the network
administrator. Subnets allow single Class A,B or C network to be used, instead of
multiple networks but still allow multiple groups of ip addresses.

Subnet Mask : A 32 bit number that numerically represents the format of an IP address,
by representing the network and subnet part with a mask bit value of 1, and host part with
a mask bit value of binary 0s.

Subnet number/ Subnet address : In IPv4 a dotted decimal number that represents all
addresses in a single subnet. Numerically smallest value in the range of numbers in a
subnet, reserved so that it cannot be used as a uni cast IP address by a host.

Zero Subnet : For every class ful IPv4 network that is subnetted, the one subnet whose
subnet number has all binary 0s in the subnet part. In decimal zero subnet can be easily
identified, because it is the same number as the classful network number.

37

Chapter 13 – Operating Cisco Routers

Physical installation
Steps required to install a router…..
Step 1. Connect any LAN cables to LAN ports
Step 2. If using an external CSU/DSU connect the router’s serial interface to CSU/DSU
and the CSU/DSU to the line from telco.
Step 3. If using internal CSU/DSU connect the routers’ serial interface to the line from
telco
Step 4. connect the router’s console port to a pc using a rollover cable as needed to
configure the router
Step 5. Connect the power cable to power port on the router
Step 6. Turn on the router

Comparison between Switch CLI and Router CLI

The configuration commands used for the following features are the same on both routers
and switches…

Similarities between switch CLI and router CLI…

• Use and Enable (privileged) mode
• Entering and exiting configuration mode, using configure terminal, end and exit
commands, and ctrl z key sequence
• Configuration of console, telnet and enable secret password
• Configuration of SSH encryption keys, and username/password login credentials.
• Configuration of host names and interface descriptions
• Speed and Duplex commands
• Shutdown and no shutdown commands to administratively disable and enable an
interface respectively
• Navigation through different configuration context modes, using commands like line
console 0 , and interface
• CLI help, command editing and command recall
• The meaning and use of start-up config in NVRAM, running config in RAM, external
servers like TFTP, along with copy commands.
• The process of reaching a setup mode either by re-loading the router with an empty
start-up config, or by using setup command

Differences….

• The configuration of IP address differ in someway between switches and routers
• Questions asked in setup mode differ

38

• Routers have an auxiliary port, intended to be connected to an external modem and
phone line, to allow remote users to dial into the router and access the CLI, by
making a phone call.

There is no show mac address-table dynamic command in a router, but a router has
show ip route command.

Router Interfaces

Routers generally have two types of physical interfaces : Ethernet Interfaces and Serial
Interfaces.

Ethernet Interfaces…
Interface Ethernet number
Interface fastethernet number
Interface gibabitethernet number

Serial Interfaces…
For PPP links and Frame relay links and uses HDLC or PPP protocols HDLC being the
default.

Valid configuration command formats….

Interface Ethernet 0
Interface fastethernet 0/1
Interface serial 1/0/1

Commands to view information about interfaces….

Show ip interface brief
Show protocols fa0/0
Show interfaces s1/0/1

Please refer to page 413 for a detailed listing example

39

Router Interface status codes and their meaning
Name Location General Meaning
Line Status First Status Refer to the layer – 1 status, eg. If the cable is
code installed, is it the right/wrong cable, is the device on
the other end powered on.
Protocol Status Second Status Refer generally to the layer – 2 status, It is always
code down if the line status is down. If the line status is up
and protocol status is down, usually is caused by
mismatch in the data link layer protocol
configurations.

Typical combinations of two interface status codes and likely reasons….
Line and protocol status Typical Reasons
Administratively down, The interface has shutdown command configured on it
down
Down, down The interface has no shutdown command configured but the
physical layer has a problem. Eg. No cable has been
connected to the interface, or with Ethernet, the switch
interface on the other end of the cable is shutdown, or the
switch is powered off.
Up, down Almost always refer to data link layer problems, most often
configuration problems. Eg. Serial link have this combination
when one router was configured to use PPP and the other
default to use HDLC.
Up, Up All is well, interface is functioning

Router Interface IP Address

Routers need an IP address on each interface.

Router1# configure terminal
Router1 <config> # interface fa0/1
Router1 <config-if> # ip address 10.1.1.1 255.255.255.0
Router1 <config-if> # ^z
Router1# show ip interface brief

Please refer to page 415 for the command output listing

Bandwidth and clock rate on serial interfaces

40

The clock rate speed sub command sets the rate in bits per second on the router that has
the DCE cable plugged into it. Show controllers serial 0/1/0 command will show if a
DCE/DTE cable is connected to a router interface. IOS accepts the clock rate command
on an interface only if it has a DCE cable attached to it, or if no cable is installed. If a
DTE cable is installed IOS silently rejects the clock rate command.

Bandwidth speed command tells the IOS the speed of the link, in kilobits, which are
mainly used by routing protocols (EIGRP, OSPF) etc in their default routing matrices.

For serial link the default bandwidth is 1544 , 1544 kbps, 1.544 Mbps, (a T1 line).
Router ethernett interface default to a bandwidth setting that reflects the current speed of
the interface. If router’s fasterethernet interface is running at 100 Mbps then the
bandwidth is 100,000 Kbps.

Clock rate uses a unit of kbps, whereas bandwidth command uses a unit of Kbps

Auxiliary Port can be configured using the
Line aux 0 command to reach the aux line configuration mode.

Summary of facts about Initial configuration Setup Mode dialogue.

• Setup mode is intended to allow basic configurations by prompting the CLI user via a
series of questions.
• You can reach the setup mode, either by booting the router after erasing the startup-
config file, or by using the setup enable mode EXEC command.
• At the end of the process you get three options (0,1,2), to either ignore the answers
and go back to the CLI (0), ignore the answers but begin again in setup mode (1), or
to use the resulting config (2).
• Ctrl C key combination to eject the user out of the setup mode
• If the user selects to use the resulting config, the router writes the configuration file to
the startup-config file as well as the running-config file.

The questions asked differ between switches and routers especially on IP configurations.

Cisco IOS Software boot sequence…
When a router first powers on, it follows these four steps…

1. The router performs Power On Self Test (POST) , to discover hardware components,
and to verify that all components work properly.

41

2. The router copies the bootstrap program from ROM into RAM, and runs the bootstrap
program

3. Bootstrap program decides which IOS (or other OS) to load into the RAM, and loads
the OS. After loading the IOS the bootstrap program hand over the control of the router
hardware to the newly loaded OS.

4. If the bootstrap program loads IOS, IOS find the configuration file typically the
startup-config from the NVRAM and loads it into the RAM as running-config.

Loading the Cisco IOS

RAM

ROM Step 2 Bootstrap

Flash

TFTP Step 3 Cisco IOS
ROM

NVRAM
Running
TFTP Step 4 Config File

Console

Router can get the Cisco IOS image from three different locations, and running config
file from other three different locations.

42

The Three Router Operating Systems
Comparison of ROMMON and RxBoot Operating Systems…
Operating Common Name Stored in Used in
Environment
ROM Monitor ROMMON ROM Older and new routers
Boot ROM Rx Boot, Boot helper ROM Only in older routers

Cisco routers use different OSs to perform some troubleshooting, to recover router
passwords, and to copy new IOS into the flash when the flash has been erased or
corrupted.

The configuration register

The configuration register is a special 16 bit number, that can be set on any cisco router.
Eg. The console speed, what IOS iamge to load etc are set using the configuration
register bits.

Config-register global configuration command sets the configuration register values.
Eg. Config-register 0x2100 sets the value to hex 2100, which causes the router to load
ROMMON OS, instead of IOS - a common practice when recovering lost passwords.

Config-register values are automatically saved, to both running config and startup-config,
but these new values will not be used until the router is re-loaded.

The show version command lists the configuration register’s current value, if different
the value that will be used once the router is re-loaded.

In most cisco routers the default configuration register setting is Hex 2102.

How routers choose which OS to load

A router chooses the OS to load based on the low order 4 bits in the configuration
register, and the details configured in any boot system global configuration command
found in start-up configuration command. The low order 4 bits, (the 4th hex digit) is the
configuration register are called boot-field.

Steps a routers uses to choose which IOS to load…

Step 1. If boot field = 0 , use the ROMMON OS
Step 2. If boot field = 1, load the first IOS file found in Flash memory
Step 3. If boot field = 2-F
a. try each boot system command in the startup-config file, in order until one works
b. if none of the boot system commands works, load the first IOS file found in the flash
memory

43

ROM

Bootstrap and
ROMMON
BOOT = 0
RAM
FLASH
IP Network

BOOT = 1
1st IOS File TFTP
ND
2 IOS File
3rd IOS File BOOT = 2...F
… NVRAM (Startup Config)
….
Last IOS File
Boot System command 1
Boot System command 2
….
Boot system command last

If all three steps fails, as it may be possible that flash memory is erased, the router send
broadcasts looking for tftp server and guessing IOS file name to load, if that fails, router
loads the ROMMON which provides the tools to recover.

From the factory Cisco routers have not boot system command configured, and comes
with a configuration register value of 0x2102, meaning boot field hex 2, the process tires
step 3, finds no boot system commands and loads the first IOS image from the flash
memory.
Boot System commands
Boot system command Result
Boot system flash The first file from flash memory is loaded
Boot system flash filename IOS with name filename is loaded from flash memory
Boot system tftp filename IOS with name filename loaded from TFTP server
10.1.1.1

44

The SHOW VERSION command

Show version command supplies a wide variety of information about the router,
including the current and future configuration register.

1. IOS Version
2 The uptime – the length of time passed since the last reload
3. The reason for the last reload of the IOS (reload command, power off/on, software
failure)
4. The time of the last loading of IOS (if the router’s clock has been set)
5. The source from which the router has loaded the current IOS
6. The amount of RAM memory
7. The number and types of interfaces
8. The amount of NVRAM memory
9. The amount Flash memory
10. The configuration register’s current and future settings (if different)

Please refer to page 430 for a detailed listing of the show version command

Bandwidth : A reference to the speed of a network link. Its origins come from earlier
communication technologies where the range or width of frequency bands dictated how
fast communication could occur.

Boot field : Low order 4 bits of the configuration register in a cisco router. The value in
the boot field in part tells the router where to look for the cisco IOS image to load.

Clock Rate : The speed at which a serial link encodes bits on the transmission medium

Configuration register : In cisco route a 16 bit use configurable value that determines how
router the router functions during initialization. In software, the bit position is set by
specifying a hex value using configuration commands.

IOS Image : A file that contains IOS – Cisco operating system that provides majority of
the router’s or switch’s features with the hardware providing the remaining features.

Power On Self Test (POST) : The process on any computer, including routers and
switches , in which computer hardware first runs hardware diagnostics on required
hardware before even trying to load a bootstrap program.

ROMMON : A shorter name of ROM Monitor , which is a low level operating system,
that can be into cisco routers for several seldom needed maintenance tasks, including
password recovery, and loading new IOS when flash memory has been corrupted.

45

RxBoot : A limited function version of IOS stored in the ROM, in some older models of
Cisco routers, for the purpose of performing seldom needed low level functions including
loading new IOS into flash memory, when flash memory has been corrupted or erased.

Configuration Command refernce…
Bandwidth kbps Interface command that sets the router’s perception
of bandwidth in kpbs
Clock rate rate Interface command that sets the speed at which the
router supplies a clocking signal, applicable when
the router has a DCE cable installed. The unit is
bit/second
Config-register value Global command that sets the hexadecimal value of
the configuration register
Boot system {file-url | filename} Global command that identifies an externally located
IOS image using a URL.
Boot system flash [flash:fs:] Global command that identifies the location of an
[filename] IOS image in flash memory
Boot system rom Global command that tells the router to load RxBoot
OS found in ROM, if one exitsts.
Boot system {rcp|tftp|ftp} Global command that identifies an external server,
filename [ip address] protocol and file name to use to load IOS from an
external server

EXEC Command reference….
Command Purpose
Show interfaces type number Lists a large set of information about each interface, or
about the one, if one is specified
Show ip interface brief List a single line of information about each interface,
including the IP address, line and protocol status, and
the method with which address was configured.
(manual or DHCP).
Show protocols type numebr Lists a single line of information about the specified
interface including ip address, line,protocol status.
Show controllers type number List many line of information per interface, or for the
specified interface, for the hardware contoller of the
interface. On serial interfaces, this command identifies
the cable as either a DCE or DTE cable.
Show version Lists IOS version as well as a lots of other usefil
information
setup Starts the setup dialogue
Copy url-from url-to Copies the file from source url to destination url
Show flash List the contents of the flash memory, including used
and available memory
reload Enable mode command to reboot the router

46

Chapter 14 – Routing Protocol Concepts and
Configuration

`
10.1.1.0/24

Fa0/0
10.1.1.251

10.1.128.251 S0/0/0 S0/1/0 10.1.130.251
Albequerque

10.1.128.0/24 10.1.130.0/24

10.1.130.253
10.1.128.252 S0/0/1
S0/0/1

10.1.2.252 10.1.3.253

` `
` `

10.1.2.0/24 10.1.3.0/24

A router adds routes to its routing table for the subnets connected to each of the router’s
interfaces. For this to occur the router must have an ip address and mask configured on
the interface, and the interface must be in an up/up status.

Show running config – will show the ip address and mask on each interface
Show ip interface brief – will show the interface status (line/protocol)

Show I p route – will show the routing table entries

10.0.0.0/24 is subnetted , 3 subnets

c 10.1.1.0 directly connected, Fastethernet 0/0
c 10.1.128.0 directly connected, serial 0/0/0
c 10.1.130.0 directly connected, serial0/1/0

terminal ip netmask-format decimal – will change the mask format to decimal

47

Static Route

Adding a static route to router alberquerque for the subnet 10.1.2.0/24 and 10.1.3.0/24

Albuquerque# configure terminal
Albuquerque(config)# ip route 10.1.2.0 255.255.255.0 10.1.128.252
Albuquerque(config)# ip route 10.1.3.0 255.255.255.0 10.1.130.253

Albuquerque# show ip route static

10.0.0.0 /24 is subnetted , 5 subnets

S 10.1.3.0 [1/0] via 10.1.130.253
S 10.1.2.0 [1/0] via 10.1.128.252

Ip route global configuration command supplies the subnet number, mask and the next
hop id address.

If the outgoing interface of a router (Albuquerque’s s0/0/0 or s0/1/0) interface is not in
up/up state the static route will not be listed in the routing table.

Extended PING - a router enable mode command allows the CLI user to change many
option on Ping command, including the source ip address, which enables to issue a ping
command from a router which resembles more closely to a ping command issues by an
end user.

Cisco ping command by default uses the output interface’s ip address as the packet’s
source address, unless otherwise specified in an extended ping.

Default Routes

Subnet 1

172.16.3.2 Subnet 2
Fa0/0
Rest of the enterprises network
S0/1
172.16.3.0
R1 R2

Subnet 3

As part of the routing process a router compares a packet’s destination ip address to its
routing table, if a no matching route is found the router discards the packet. A default

48

route is a route that matches all destination ip addresses, and the router forwards using the
default route, when no other route is matched with packets destination ip address.

In the above example the router R1 can be configured in three ways…..

• Configure hundreds of static routes on R1, but all with an outgoing interface of s0/1
and next hop id address 172.16.3.2 (R2).
• Enable routing protocols on routers to learn routes
• Add a default route on R1 with outgoing interface as S0/1

R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.3.2

R1# show ip route

172.16.0.0/24 is subnetted, three subnets

C 172.16.3.1 directly connected fa0/0
C 172.16.3.2 directly connected s0/1

S* 0.0.0.0/0 [1/0] via 172.16.3.2

S* - statically configured default route

49

Routing Protocol Overview

RIP2 – Basic Concepts.

Example of how RIP-2 advertises routes

`
3
R3 IP Routing Table
5 172.16.5.253 Fa0/0 Subnet Out.Int Next hop Metric
------------------------------------------------------------
I have a route to
172.16.3.0 s0/1 172.16.6.252 1
172.16.3.0/24 , Metric 2

S0/0 S0/1

R3
2
5 I have a route to
I have a route to 172.16.3.0/24 , Metric 1
172.16.3.0/24 , Metric 2

S0/1 172.16.6.252
172.16.2.252 S0/1
S0/0
S0/0
172.16.1.251
R1 2 R2
Fa0/0 Fa0/1 172.16.3.252 1
I have a route to
172 .16.3.0/24 , Metric 1

` `
` `

4
R1 IP Routing Table
Subnet Out.Int Next hop Metric
------------------------------------------------------------
172.16.3.0 s0/0 172.16.2.252 1

1. Router r2 learns a connected route for subnet 172.6.3.0
2. R2 sends routing updates to its neighbors’ listing subnet, mask and a distance , metric
3. R3 hears the routing updates and add a route to its routing table for the subnet
172.16.3.0/24 with R2 as the next hop router.
4. At the same time R1 also hears the routing update sent directly from R2 to R1 and adds
the route to its routing table
5. R1 and R3 then send routing updates to each other for the subnet 172.16.3.0/24 with
metric value 2.

RIP routers send periodic routing message about every 30 seconds.

50

Interior and Exterior Routing Protocols

Interior Gateway Protocol (IGP) : A routing protocol that was designed and intended
for use inside a single autonomous system

Exterior Gateway Protocol (EGP) : A routing protocol that was designed and intended
for use between different autonomous systems.

BGP – Border Gateway Protocol is the used to exchange routes between routers in
different autonomous systems and is an EGP.

ICANN – assigns an ASN – Autonomous System Number

Routing protocols classes/algorithms and Protocols that use them
Class/Algorithm IGPs
Distance vector RIP-1, RIP-2, IGRP
Link-state OSPF , Integrated IS-IS
Balanced Hybrid (also called advanced EIGRP
distance verctor)

Metrics

Each routing protocol defines a metric that gives an objective numeric value to the
goodness of each route. The lower the metric the better the route.

RIP uses a metric called hop count, which counts the number of routers (hops) between a
router and a subnet.

EIGRP uses a metric which by default considers both the interface bandwidth and
interface delay settings as input into the mathematical formula to calculate the metric.

ROUTERS generally perform routing more quickly with smaller routing table, Route
Summarization (Auto summarization and Manual Summarization) helps shorten the
routing table while retaining all the needed routes in the network.

51

Routing protocol that must consider Class (A,B,C) rules are called Classful Routing
Protocols, and that do not need to consider Class rules are called Classless Routing
Protocols.

Comparing Classful and Classless Routing Protocols

Feature Classless Classful
Support VLSM Yes No
Send subnet masks in routing updates Yes No
Support manual route summarization Yes No

The process used by routing protocols to recognize changes in a network (a link comes
up or fail, a router is added or removed), to figure out now-best route to each subnet, and
to change each routers routing table is called Convergence.

Summary of Interior Gateway (routing) Protocols
Feature RIP-1 RIP-2 EIGRP OSPF IS-IS
Classless No Yes Yes Yes Yes
Supports VLSM No Yes Yes Yes Yes
Sends mask in updates No Yes Yes Yes Yes
Distance Vector Yes Yes No No No
Link-state No No No Yes Yes
Support No Yes Yes No No
autosummarization
Support Manual No Yes Yes Yes Yes
Summarization
Proprietary No No Yes No No
Routing updates send to No Yes Yes Yes n/a
a multi cast ip address
Support Authentication No Yes Yes Yes Yes
Convergence Slow Slow V.Fast Fast Fast

Configuring and Verifying RIP-2

RIP-2 Configuration

Step 1. Router RIP configuration command to move into the RIP configuration mode
Step 2. Version 2 RIP subcommand to tell the router to use RIP Version 2
Step 3. Use one or more network net-number to enable RIP on the correct interface
Step 4. passive-interface type number to disable RIP on an interface

The RIP network command uses a classful network number as its net-number parameters.

52

For any of the router’s interface ip address within that clasful network, the router does the
following three things…

• The router multicast routing updates, to a reserved IP multicast address 224.0.0.9
• The router listens for the incoming routing updates on the same interface
• The router advertises about the subnet connected to the interface

Sample RIP configuration…..

10.1.1.2.

Fa0/0
199.1.1.1
S0/0

Fa0/1 S0/1 199.1.2.1
R1

10.1.4.2.

R1# configure terminal
R1(config)# router rip
R1 (config-router)# version 2
R1(config-router)# network 199.1.1.0

S0/0 – 199.1.1.1. is in a class C network 199.1.1.0
S0/1 – 199.1.2.1. is in a class C network 199.1.2.0
Fa0/1 and fa0/0 10.1.4.2. and 10.1.1.2 are in class A network 10.0.0.0

To disable RIP on fa0/1 interface….

R1(config-router)# passive-interface fa0/1

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RIP-2 Verification

RIP Operational commands…
Command Purpose
Show ip interface brief List one line per interface, including ip address and interface
status; an interface must have an ip address and up/up status
for the RIP protocol to work
Show ip route [rip] List the routing table, including RIP learned routes, and
optionally just RIP learned routes
Show ip protocols Lists information about the RIP configuration, plus IP
addresses of neighbouring RIP routers, from which local
router has learned the routes.

Show ip route
Show ip route rip
Show ip route 10.1.2.1

Please refer to page 460 for a detailed listing of show ip route command.

Show ip route … listing contains…..

• Subnet number with the mask at the heading line
• Next hop router’s ip address
• Local router’s outgoing interface
• Length of time since the router heard a routing update about this route
• The RIP metric for this route, second number in the square brackets [120/1]
• Administrative Distance of the route first number in the bracket [120/1]

Administrative Distance : is numeric number representing the routing protocol, which is
used to determine the lower number (best route) in a network that uses multiple routing
protocols.

IOS defaults for administrative distance
Route Source Administrative Distance
Connected Route 0
Static routes 1
EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP (V1 and V2) 120

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Unknown or unbelievable 255

Normally a static route has lower administrative distance than a RIP learned route,
however a backup static route can be configured with an administrative status higher
than any default administrative distances say 150, so that this static route will be added to
the routing table only if no route is learned by any routing protocols.

Show ip protocol
Please refer to page 464 for a detailed listing…..

Q. How it can be used to trouble shoot RIP problems???
Ans. By checking the Version Information and Routing Information Sources of the show
ip protocol command output. It is possible that one Router is configured as RIP Version 2
and other one default Version 1. Also you could verify the router is getting RIP messages
from all the expected sources in the network.

Examining RIP messages using debug

Debug ip rip
Undebug all
Show process
Service timestamps

Please refer to page 465 for a detailed listing

RIP2 routing messages are sent to multicast IP address : 224.0.0.9
RIP1 routing messages are sent to broadcast IP address : 255.255.255.255

Definitions…

Administrative Distance : In cisco routers a mean for a router to choose between
multiple routes to reach the same subnet, when those routes learned by different routing
protocols. The lower the administrative distance, the better the source of the routing
information.

Autonomous Systems : An internetwork in the administrative control of a single
organization., in side which that organization typically runs a Interior Gateway Protocol
(IGP).

Backup Static Route : A static route configured with an administrative status higher than
any of the default routing protocol administrative statuses.

55

Balanced Hybrid : A general type of routing algorithm, other than distance vector and
link state, EIGRP is the only routing protocol that uses Balance Hybird algorithm.

Classful Routing Protocol : Does not transmit mask information along with the subnet
number, and therefore must consider Class A, B or C network boundaries, and perform
autosummarization at those boundaries. Does not support VLSM.

Classless Routing Protocol : An inherent characteristic of a routing protocol, specifically
that routing protocols send subnets masks in it routing updates, thereby removing any
need to make assumptions about the addresses in a particular network or subnet. Making
it able to support VLSM and manual route summarization.

Covergence : Time required for a routing protocol to react to a change in the network,
removing bad routes, and adding new, better routes so that the current best routes are in
all routers’ routing tables.

Default Route : On a router, the route that is considered to match all packets that are not
otherwise matched some more specific routes.

Distance Vector : Logic behind some of the Interior Routing Protocols such as RIP.
Distance Vector algorithm calls for each router to send its entire routing table in each
update, but only to its neighbours. Distance vector algorithm can be prone to routing
loops but are computationally simpler than the link state algoritm.

Exterior Gateway Protocol (EGP) :A routing protocol that was designed to exchange
routing information between two different autonomous systems.

Interior Gateway Protocol (IGP) : A routing protocol designed to use within an
organization (autonomous system).

Link State : A classification of underlying algorithm used in some routing protocols. Link
state protocols build a detailed database that lists links (subnets) and their states
(up/down) from which the best route is calculated.

Metric : A unit of measure used by routing protocol algorithms, to determine the best
route, for the traffic to use to reach a particular destination, in RIP-2 it is the hop-counts.

Routing Updates : A generic reference to any routing protocol message, in which it sends
the routing information to its neighbours.

Variable Length Subnet Masking (VLSM) : The capacity to specify different subnet
masks for the same Class A,B, or C network number on different subnets. VLSM can
help optimize available address space.

56

Configuration command reference…
Command Description
Router rip Global configuration command that moves the user
to the RIP configuration mode.
Network network-number RIP subcommand that lists a clasful network
number, enabling RIP on all of that router’s
interfaces in that classful network.
Version {1|2} RIP subcommand that sets the RIP version
Passive-interface [default] RIP subcommand that tells the RIP to no longer
interface type, number advertise RIP updates on the listed interface
Ip address ip-address mask Interface subcommand that sets the routers inteface’s
ip address and mask
Ip route prefix mask {ip-address | Global command that defines a static route
interface type, interface number}
(eg. Ip route 10.102.0
255.255.255.0 10.1.128.252)
Service timestamp Global command that tells the router to put a
timestamp on log messages, including the debug
messages

EXEC command reference…
Command Description
Show ip interface brief Lists one line per router interface, including ip
address and interface status
Show ip route [rip|static| Lists the routing table
connected]
Show ip route ip-address List the details of the routes the router would match
for a packet sent to the listed ip-address
Show ip protocols Lists information about RIP configurations, plus the
IP address of the neighbouring routers from which
the local router has learned routes.
Show process Lists information on processes running in IOS, and
also overall CPU utilization status
Terminal ip netmask-format For the length of the user session, causes the router
decimal to display mask information in dotted decimal
instead of prefix format
Debug ip rip Tells the router to generate detailed message logs for
each send and received RIP updates.

57

Chapter 15 – Troubleshooting IP Routing
IP Troubleshooting Tips and Tools

Avoiding reserved IP addresses

• Addresses that are always reserved
• Two addresses that are reserved in each subnet (subnet number and subnet broadcast
address)
• Addresses in two special subnets of each classful network, namely zero subnet and
broadcast subnet

First octet values of addresses that are always reserved, and that cannot be assigned to
hosts. Reserved IP addresses….as recognized based on the value of the first octect….
• 0 (because network 0.0.0.0 is always reserved)
• 127 (because network 127.0.0.0 is always reserved)
• 224 – 239 (all Class D – Multicast IP Addresses)
• 240 – 255 (all Class E – Experimental IP Addresses)

Summary of reasons why an exam question should or should not allow the use of the zero
and broadcast subnets. Determine whether a question allows the use of the Zero and
Broadcast subnets
Clue in the question Subnet reserved?
Says nothing about it (default for the exam) No
List the ip subnet-zero configuration command No
Use a classless routing protocol (RIP-2, EIGRP, OSPF) No
List the no ip subnet-zero configuration command YES
Use a classful routing protocol (RIP-1) YES

Summary of 4 tips when approaching IP Addressing related questions on the exam…

• Check the mask used on each device in the same LAN; if different then the devices
cannot have the same view of the range of addresses in the subnet.
• On a point-to-point WAN link, check the IP addresses and masks on both end of the
link, and confirm that the two ip address are in the same subnet.
• When checking to confirm that hosts are in the same subnet, do not just examine the
subnet number. Also check the subnet mask and the implied range of IP addresses.
• Be ready to quickly use the commands in the table below, to find the ip address,
masks, and subnet numbers.

58

Host Networking

Summary of how hosts think about routing, address assignment, name resolution and
ARP…

Routing : If the packet’s destination is in the same subnet, send the packet directly, if not
send the packet to the default gateway.

Address Assignment : Before sending any packets, the host may use DHCP client
services to learn its IP address, mask, default gateway, and DNS ip address. The host
could also be statically configured with the same details.

Name resolution : When the user directly or indirectly when a host references a host
name, the host typically uses DNS name resolution request to ask DNS to identify the
host’s ip address, unless the host already has the information in its name cache.

IP-to-MAC resolution: The host uses ARP requests to find the other host’s MAC
address, or the default gateway’s ip address, unless the information is already in the
host’s ARP cache.

Command Function
Ipconfig/all Displays detailed IP configuration information for all
interface, including IP address, mask, default gateway, and
DNS IP address.
Ipconfig/release Releases any DHCP leased IP addresses
Ipconfig/renew Acquires an IP address and related information using DHCP
Nslookup name Sends a DNS request for the listed name
Arp –a Lists the host’s ARP cache
Ipconfig/displaydns List host’s name cache
Ipconfig/flushdns Remove all dns-found name cache entries
Arp -d Flushes (removes) the host’s ARP cache
Netstat -rn Displays host’s routing table

Troubleshooting Host Routing Problems.
Two typical reasons why a hosts cannot ping other hosts in the same subnet.
If a ping of a same subnet host fails, the root cause typically falls into two categories…

• The two hosts have incorrect ip address, and mask configuration, so that at least one
of the two hosts thinks it is in a different subnet.
• The two hosts have correct ip address and mask configuration, but the underlying
Ethernet has a problem

59

When a host can ping hosts in the same subnet, but not the hosts in a different
subnet…..the root cause could be…..

• There is a mismatch between the host’s default gateway configurations and the router
acting as the default gateway. The problems include mismatched masks between
hosts and the router, which impacts the perceived range of addresses in the subnet, or
the host simply referring the wrong router IP address.
• If the default gateway settings are correct but the ping of the default gateway still
fails, then probably there is a layer 1 or 2 problem in the LAN.
• If the default gateway setting are correct, and ping works, but if ping of one of the
other router interface fails, then the router’s other interface must have failed.

Tips regarding how a router matches a packet’s destination IP address as part of the
routing process. Finding the matching route on a router…key points to remember…..

• When a destination IP address matches more than one route in a router’s routing
table, the router uses the most specific route, the route with the longest prefix length.
• Although the router uses binary maths to match the destination IP address to the
routing table entries, you can simply compare the destination address to each subnet
in the routing table. If a subnet’s implied address range include the packet’s
destination address, the route matches packet’s destination.
• If the question include a simulator, you could easily find the matched route by issuing
a command show ip route address command, which lists the route matched for the IP
address listed in the command.

Please see a listing of show ip route rip , in page 484 , and determine the matching route
for a list of destination ip address from page 487.

Troubleshooting Commands

Show ip arp – lists the contents of a router’s ARP cache.

R1# show ip arp

Protocol address age(min) hardware address type
Interface
Internet 172.161.1.1 8 0013.197b.2f58 ARPA Fa0/1

Please refer to command output in page 485

60

It lists the ip-address, mac address and interface. The age heading if lists a number, the
value represents the number of minutes since the router last received a packet from the
host and the timer is reset each time a matching packet is received. If the age heading lists
a – the arp entry represents the ip address assigned to the router.

Traceroute command list the ip addresses of the routers in the route
Please refer to the listing on page 486

telnet newYork

Ctrl + Shift + 6 and x (to exit/suspend a telnet session

show sessions
where (lists all the suspended telnet sessions)

resume 1 (resumes telnet session 1)
disconnect 1 (disconnects telnet session 1)

show users (lists the users logged in to the telnet session)

Routing Troubleshooting Scenarios….please refer to page 491 through to 505.

Reminders that are helpful when thinking about source and destination MAC, and IP
addresses that are used at various points in an internetwork.

• IP packet flows from the sending host to the destination host
• The data link header and trailer that encapsulates the ip packet, do not flow over the
complete end-to-end route-instead each data link helps move packet from a host to a
router, between routers or from a router to a destination host.
• For the process to work, the data link destination lists the next device’s data link
address
• The IP header lists the sending host’s ip address, and destination hosts IP address, and
never changes throughout the journey through the route
• Routers discard the received data link header and trailer for received frame, and build
a new header and trailer-appropriate for the outgoing interface-before forwarding the
frame.
• On LANs, hosts and routers use ARP to discover Ethernet MAC address used by
other devices on the same LAN
• On Point-to-point WAN links ARP is not required, and data link addressing is
uninteresting and can be ignored.

61

Two key requirements for a router to add a connected route…

• The interface’s two status codes are up and up
• The interface has an IP address correctly configured

Command Purpose
Show sessions Lists the suspended telent and ssh sessions, from which the telnet
and ssh session were created
Where Does the same thing as show sessions
telnet { hostname| ip Connect the CLI to another host using telnet
address}
Ssh –l username Connects CLI to another host using ssh
{hostname | ip
address}
Disconnect Disconnects currently suspended telnet or ssh session, based on
[connection number] the connection number as seen with the show sessions command
Resume Connects CLI to a currently suspended telnet or ssh session, based
[connection number] on the connection number as seen with the show sessions
command
Traceroute Discovers if a path from a router to a destination ip address is
{hostname|ip working, listing each next hop router in the route.
address}
Ctrl + Shift + 6 and The key sequence required to suspend a telnet or ssh session
x
Show ip arp Lists the contents of router’s arp cashe
Show arp Lists the contents of router’s arp cashe
Show ssh Lists the information about the users logged into the router using
ssh
Show users Lists the information about the users logged into the router,
including telnet, ssh , and console users.

62

Ccent notes part 3

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