This document discusses subnetting and classless inter-domain routing (CIDR) in TCP/IP networks. It defines subnetting as dividing a network into subnets using parts of the host ID. CIDR is introduced to avoid depletion of class B addresses by subnetting them and assigning as class C addresses. It also folds network IDs to simplify routing table entries. Supernetting is discussed as using a subnet mask to represent multiple classful network IDs with a single entry in internet router tables.

1. Chapter
Subnets in TCP/IP Networks
© N. Ganesan, Ph.D.

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Chapter Objectives

3. Module
Introduction to Subnetting

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Subnetting
• Division of a network into subnets
– For example, division of a Class B address
into several Class C addresses
• Some of the host IDs are used for
creating subnet IDs

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Need for Subnetting
• Classes A and B have a large number of hosts
corresponding to each network ID
• It may be desirable to subdivide the hosts in
Class C subnets
• Often, there is a limitation on the number of
hosts that could be hosted on a single
network segment
– The limitation may be imposed by concerns
related to the management of hardware
• Smaller broadcast domains are more efficient
and easy to manage

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Subnetting Principle
• Use parts of the host IDs for subnetting
purpose
• A subnet mask is used to facilitate the
flow of traffic between the different
subnets and the outside network (hops)
– A hop is the distance a data packet travels
form one node to the other

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Using Host IDs to Subnet
140 15 0 0
Class B Network
140 15 1 0
140 15 2 0
Subnet 1
Subnet 2
Third octet is now used for subnet IDs
140 15 3 0
Subnet 3

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Subnet Configuration
140 15 1 0
140 15 1 1
Subnet ID
First Host ID
140 15 1 254…..
Last Host ID

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Routing of Traffic
140.15.0.0
140.15.2.0
140.15.1.0
140.15.3.0
Routing
Subnets
1
2
3
Outside world

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End of Module

11. Module
Subnetting Example

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Subnetting Example
• Consider the case of a class C address
195. 175.25.0 assigned to an
organization
• Subnets can be constructed by
allocating part of the higher-order bits
of the host ID
• Assume that three of the higher-order
bits of the host ID are to be reserved for
that purpose

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Subnetting Structure
195 175 25 0
11100000
Subnet Mask

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Sub Net Last Octet Subnet ID
1 00000000 195.175.25.0
2 00100000 195.175.25.32
3 01000000 195.175.25.64
4 01100000 195.175.25.96
5 10000000 195.175.25.128
6 10100000 195.175.25.160
7 11000000 195.175.25.192
8 11100000 195.175.25.224
Usable
Subnets
(6)

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Sample Subnet Division
Router
195.175.25.32 195.175.25.64
195.175.25.33
.
.
.
195.175.25.62
195.175.25.65
.
.
.
195.175.25.94
30 hosts per subnet.
Subnet 2Subnet 1

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Total Number of Subnets and
Hosts
• All zeros and ones are not used
– This has been overcome in the new RFC
• Total number of subnets is 6
• Number of hosts per subnet is 30
• Subnet mask is 255.255.255.224
– 255.255.255.11100000

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End of Module

18. Module
The Routing Process

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Overview of the Masking Process
• IP address and subnet masks are used
for the masking operation
• The purpose of masking is to identify
whether an IP address corresponds to a
local host or a remote host
• The mathematical technique used is
known as the ANDing process

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ANDing Process
• Similar to the AND Boolean operator
• Consider A = B and C
– A is true only when B and C are true
– Otherwise, A is false for all other scenarios

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ANDing Table
B C B AND C
0 0 0
0 1 0
1 0 0
1 1 1

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Subnet Masking
• AND host IP and subnet mask value at
startup to identify network ID
• AND destination IP address and subnet
mask value determine either of the
following:
– IP represents local host
– IP represents remote host

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Subnet Masking Example
• Subnet ID: 195.175.25.32
• Subnet Mask: 255.255.255.224
• Host address
– 195.175.25.34
• Case 1 destination address
– 195.175.25.40
• Case 2 destination address
– 195.175.25.67

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Network Scenario
Router Subnet Mask: 255.255.255.224
Host
195.175.25.34
Local Host
195.175.25.40
Outside
World
195.175.25.40
195.175.25.67

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Computing Subnet ID at Startup
Host ID 195 175 25 34
11000011 10101111 00011001 00100010
Subnet
Mask
255 255 255 224
11111111 11111111 11111111 11100000
ANDing
Result
195 175 25 32
11000011 10101111 00011001 00100000
Yields subnet ID.

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TCP/IP
Properties
of the Host

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Masking of Destination
Address:Case 1
Destinati-
nation IP
195 175 25 40
11000011 10101111 00011001 00101000
Subnet
Mask
255 255 255 224
11111111 11111111 11111111 11100000
ANDing
Result
195 175 25 32
11000011 10101111 00011001 00100000
Yields subnet ID to be that of the local subnet.

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Case 1 Forwarding of Data
Packets
• The destination host is local
• Broadcast for the hardware address of
the local host at IP 195.175.25.40
• Send information to the local host

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Masking of Destination
Address:Case 2
Destinati-
nation IP
195 175 25 67
11000011 10101111 00011001 01000011
Subnet
Mask
255 255 255 224
11111111 11111111 11111111 11100000
ANDing
Result
195 175 25 64
11000011 10101111 00011001 01000000
Yields subnet ID to be that of different subnet.

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Case 2 Forwarding of Data
Packets
• The destination host is remote
• Send information to the gateway
• The router at the gateway will route the
data packet to the appropriate subnet

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Gateway IP
address
specified
In TCP/IP
properties.

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Summary of Transmission and
Routing of Data Packets
Router Subnet Mask: 255.255.255.224
Host
195.175.25.34
Local Host
195.175.25.40
Subnet at
195.175.25.64
195.175.25.40
(Case 1)
195.175.25.67
(Case 2)

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Valid Subnet Masks for Class C
Addresses
Subnet Mask Subnets Hosts Host Total
255.255.255.192 2 62 124
255.255.255.224 6 30 180
255.255.255.240 14 14 196
255.255.255.248 30 6 180
255.255.255.252 62 2 124
255.255.255.254
255.255.255.255

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End of Module

35. Module
Subnetting Convention

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Subnet Convention
• Consider the following Class C example
– 195.175.25.0/27
• In the above case, the first three octets
and the first three higher-order bits of
the fourth octet are used in subnet
masking
– 3*8+3 = 27 bits from the beginning of the 32
bit IP address

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Subnet Convention Illustrated
11111111 11111111 11111111 11100000
8 Bits 8 Bits 8 Bits 3 Bits
Total number of masking bits = 27
195.175.25.0/27
Network ID

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Variable Length Subnets
Source:
Microsoft White
Paper
135.41.0.0/16
135.41.0.0/17 135.41.128.0.0/17
135.41.128.0/21
135.41.128.0./17
135.41.248.0/21
135.41.248.0/24
1 Network, 32,766 hosts
15 Networks, 2046 hosts per network
135.41.255.0/21
Subnet
Subnet
8 Networks, 254 hosts per network

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End of Module

40. Module
Classless Inter-Domain Routing
(CIDR)

41. © N. Ganesan, All
rights reserved.
Classless Inter-Domain Routing
(CIDR)
• To avoid the depletion of the class B
addresses, it is subnetted and assigned
as class C addresses
• To avoid the proliferation of network
IDs that would complicate entries in the
routing tables, they were folded for
easing the routing process
• The above is known as CIDR

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Subnetting of Class B Example
• Consider the requirement of 2000 hosts by a
company
• Allocation of one class B network ID would
yield 65,534 hosts
– Far more than required
• The solution is to subnet a B class address
– 8 C class network IDs with each network being
able to support 254 hosts
– The total number of hosts supported is 2,032

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CIDR
• CIDR enables the folding of network IDs
• The Internet router tables will need one entry
for network ID with the use of a subnet mask
for supernetting
– Otherwise, the table need to carry 8 entries in the
previous example
• RIP for IP version 2, OSPF and BGPv2 are
protocols that support CIDR

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Classless Addressing
Fixed Variable
Fixed + Zeros
Fixed + Variables
Network ID
Host IDs

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End of Module

46. Module
Supernetting

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Supernetting and CIDR
220.78.168.0
Source: Microsoft White
Paper on TCP/IP
Network ID
220.78.168.0
Subnet Mask
255.255.248.0
(For supernetting)
220.78.175.0
220.778.168.0
Internet Router Entry
Network ID
.
.
.
8 Network IDs

48. End of Module
End of Chapter