Understanding IPv4 Addressing and Address Classes

Understanding IPv4 Addressing and
Address Classes
IPv4 has been in use since the start of the Internet,
and is widely deployed across the Internet, and
home networks
In the lecture you will learn
IPv4 address structure
IPV4 Address classes
Special and reserved IP addresses
Broadcast basics

Address Classes
IPv4 uses 32 bits for addressing
The 32 bits are split into 4 bytes and each byte is
separated by a dot(.)
So it is of this form: a.b.c.d
Where the value of a,b,c or d is between 0-255
decimal
 A typical IP address appears like this: 192.168.0.1
Networks and Nodes
 An IP address has two components: - A network
component, and a node component.

Address Classes
 As an Analogy if you think of the house address: it is of the
form: House Number + Street name e.g 12 King Street.
 For computer networks the network number is equivalent
to the street name and the house number is the Node
Address
 The earlier implementation of IPv4 used address classes to
divide the address space into network and node components
 This arrangement was very wasteful of IP addresses and was
discontinued, but the terms Class A, B and C networks are still
used

Address Classes
IPv4 Address Classes
The address classes divide the address space into
addresses that support:
 Large numbers of nodes – Intended for a large
organization – Class A addresses
 Medium number of nodes- Class B addresses
 Small number of nodes- Intended for a small organization
– Class C addresses
 IP addresses reserved for Multicast- Class D addresses
 IP addresses reserved for experimental purposes only (
R&D or Study) - Class E addresses

Address Classes
 For Example Class A addresses would be used by large
organizations (e.g. IBM) which had lots of computers
(nodes) and so would require a large number of node
addresses.
 Because there would only be a small number of large
organizations then there would only be a small number of
class A networks
 A class A address uses 8 bits for the network Address
and 24 bits for node addresses. We can write this as:
Net.Node.Node.Node
 Therefore there can only be 256 (28) Class A
networks but each network can have 16,777,216
(224) nodes.

 Class B network addresses were for medium sized
organizations and used 2 bytes (16 bits) for the
Network and 2 bytes for node addresses. We can write
this as:
Net.Net.Node.Node
 Class C network addresses were for small organizations
and used 3 bytes for the Network and 1 byte
for node addresses.
Net.Net.Net.Node
 The table on the next slide shows the summary of the
distribution between Network and Node IPs

Address Classes

Address Classes
How to Distinguish IP Address Classes
 We need a way of distinguishing a class A address from a
Class B ,C,D or E address
 The method used was to use the location on the first 0 bit
in the most significant bits of the first byte.
Class A
 If the first bit is 0 then we have a class A Address
 The other 7 bits can be either 0 or 1 (shown as X)
 This means that a class A network address is always in the
range 0 to 127 – all zeros 00000000, and all ones –
01111111 except first 0

Address Classes
Class B
 With a Class B address the first bit is a 1 and the next one
is a 0
 This means that a class B network address is always in the
range 128 to 191 – 1000000 and 1011111
Class C
 With a Class C address the first two bits are 1’s and the
next one is a 0
 This means that a class C network address is always in the
range 192 to 223 – 1100000 and 11011111

Address Classes
Class D
 Class D addresses have their first three bits set to “1” and their
fourth bit set to “0”
 This means that a class C network address is always in the
range 223 to 239 – 1110000 and 11101111
 Class D addresses are used for multicasting applications
 Multicasting means to transmit a single message to a select
group of recipients.
 A simple example of multicasting is sending an e-mail message
to a mailing list
 Teleconferencing and videoconferencing also use multicasting

Address Classes
 Note that multicasting refers to sending a message to a select
group whereas broadcasting refers to sending a message to
everyone connected to a network
 In multicasting data is not destined for a particular host, that is
why there is no need to extract host address from the IP
address, and Class D does not have any subnet mask.
Class E
 Class E networks are defined by having the first four network
address bits as 1
 That encompasses addresses from 240.0.0.0 to
255.255.255.255 – 11110000 to 11111111
 This IP Class is reserved for experimental purposes only (R&D or
Study)

Address Classes
 Like Class D, this class too is not equipped with any subnet
mask.
 This type of addressing is known as classful addressing
and resulted in very wasteful IP address allocation.
 It was replaced by a newer method called Classless Inter-
Domain Routing (CIDR)

Address Classes
Public, Private and Special Addresses
All IPv4 IP addresses can be divided into three major
groups
 Global, or public, or external 0r 'WAN addresses' — those that
are used in the Internet
 Private, or local, or internal addresses or ‘LAN addresses’ —
those that are used in the local network (LAN)
 Special addresses – these are set aside for specific uses

Address Classes
Public IP addresses
 It is a public global address that is used on the Internet
 It is assigned to every computer that connects to the
Internet where each IP is unique
 A public IP address can be either static or dynamic
 A static public IP address does not change and is used
primarily for hosting webpages or services on the Internet
 A dynamic public IP address is chosen from a pool of
available addresses and changes each time one connects
to the Internet (this is provided by the ISP)
 Most Internet users will only have a dynamic IP assigned
to their computer which goes off when the computer is
disconnected from the Internet.
 Thus when it is re-connected it gets a new IP

Address Classes
 Public IP addresses will be issued by an Internet Service
Provider
 They ranges from 1 to 191 in the first octet, with the exception
of the private address range established below
 Private IP addresses
 An IP address is considered private if the IP number falls within
one of the IP address ranges reserved for private networks such
as a Local Area Network (LAN)
 The Internet Assigned Numbers Authority (IANA) has reserved
the following three blocks of the IP address space for private
networks (local networks)
 Class A: 10.0.0.0 – 10.255.255.255
 Class B: 172.16.0.0 – 172.31.255.255
 Class C: 192.168.0.0 – 192.168.255.255

Address Classes
 Private IP addresses are used for numbering the
computers in a private network
 This includes home, school and business LANs in
airports and hotels which makes it possible for the
computers in the network to communicate with each
other
 Devices with private IP addresses cannot connect directly
to the Internet
 If the private network is connected to the Internet
(through an Internet connection via ISP) then each
computer will have a private IP as well as a public IP
 It’s important for technicians to understand that IT
personnel can choose to use any of the private address
ranges for their LAN devices

Address Classes
 It is not at all uncommon for a technician to be confronted
with a client’s network where the local addresses are in
the range of 10.0.0.(1-254), and the subnet mask used is
255.255.255.0
 This is an example of using Class A private addresses with
a Class C subnet, which makes this a Class C network
 It is the subnet mask that defines which “class” a LAN
network’s addressing is using

Subnetting
What is Subnetting?
 Sub-netting allows you to create smaller network (sub
networks; subnets) inside a large network by borrowing
bits from the Host ID portion of the address
 We can use those borrowed bits to create additional
networks, resulting in smaller-sized networks
 Suppose I want to build a network that will support up to
30 devices in different segments.
 Without sub-netting, I will need four (4) Class C networks
to support this design. For example:
 Network #1: 192.168.1.0
 Network #2: 192.168.2.0
 Network #3: 192.168.3.0
 Network #4: 192.168.4.0

Sub-netting
 Each of these networks will support 254 IP addresses
leading to a wastage of (254 * 4) – (30 * 4) IP addresses
i.e. 896 IP addresses!
 If you look at the design requirement of 30 hosts per
network, you will discover that I only need 5 bits in the
host ID portion of a Class C network to satisfy my
requirement
 This means I still have 3 bits unused that I can use those
three bits to create smaller networks
 For this example, let’s take the 192.168.1.0 network

Subnetting
 By borrowing 3 bits, I can create 8 subnets:
 192.168.1.0
 192.168.1.32
 192.168.1.64
 192.168.1.96
 192.168.1.128
 192.168.1.160
 192.168.1.192
 192.168.1.224

Subnetting
 These subnet addresses probably look weird to you – they
look like normal IP addresses. However, looking at them in
their binary form makes things clearer:

Subnetting
 With subnetting, not only have we used only one Class C
network, we have created 8 subnets from that network,
each one supporting up to 30 hosts!
 We can use 4 of these subnets for our network and
reserve the remaining 4 subnets for future expansion
 This results in great waste reduction – from 896 wasted IP
addresses to 120 reserved IP addresses
Subnet Masks
 With what we have done, we have created a problem for
computers and other networking devices: how are they
supposed to differentiate between a subnet 192.168.1.32
and an IP address 192.168.1.32?
 This is where subnet masks (also called network masks)
come in.

Subnetting
 A subnet mask is the representation of the network
portion of an address.
 It is also made up of 32 bits with all the bits that represent
the network portion being marked as 1s and the other
parts marked as 0s
 For example, the subnet masks of the IP address classes
are:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
 Therefore, a Class C network of 192.168.1.0 can be
represented as: 192.168.1.0 255.255.255.0.

Subnetting
 Note: It can also be represented using prefix length (CIDR)
notation where only the 1s that make up the network
portion are counted and represented with a slash
e.g. 192.168.1.0/24.
 With subnetting, the borrowed bits from the host ID are
counted as part of the network bits.
 So if we revisit our example above again, the 192.168.1.32
subnet can be represented as 192.168.1.32
255.255.255.224 (or 192.168.1.32/27)
 By comparing the “turned on” bits (i.e. 1s) in the
subnet mask to an IP address, a network device can
determine what network a particular IP address
belongs to

Subnetting
 For example, the 172.17.250.145 IP address with a subnet
mask of 255.255.248.0 belongs to the 172.17.248.0
255.255.248.0 subnet

Subnetting
A Note about CIDR
 So far, we have talked about subnetting in terms of IPv4
address classes.
 This was just to help with understanding – most networks
today are classless.
 In a bid to slow down the exhaustion of IPv4 addresses
and also reduce the size of the Internet routing table, the
IETF introduced Classless Inter-Domain Routing (CIDR) in
1993 which basically did away with classes
 So with CIDR, we just have a network represented by a
network address and a prefix length e.g. 192.45.96.0/22.
 Note: In the CIDR example I used above (192.45.96.0/22),
this address block will be seen as Class C in a classful
network.

Subnetting
Why do we need subnetting?
Now that we have seen what subnetting is, let us
consider some of the reasons we create subnets:
 Reduce wastage
 As we have already seen, subnetting (and CIDR on a larger
scale) helps us conserve both public and private IP
addresses
 Improve Network Performance
 The larger a network is, the busier (more congested) it is.
With subnetting we create small network thereby
increasing their performance (easy to manage)

Subnetting
 Isolation
 With smaller networks, you are able to isolate effectively as
faults inside one subnet will not necessarily spread into
other subnets
 This is also important during security incidents so that even
if one subnet is affected, the entire network is not brought
down
 Easier administration
 Subnetting, when done properly, can make network
administration more effective.
 For example, a multinational organization can design their
network in such a way that each region is assigned an IP
address block from a larger address block and subnetting is
used within regions to further divide the blocks among
networks

Subnetting
Minimum subnet size to accommodate a number of
hosts
 You need to be able to design networks in such a way that
there will be enough IP addresses for the devices that will
be used on the network
 Of course, you can always go for a large address block
(e.g. /8) but like we already established, using smaller-
sized subnets is more efficient
 As such, you must be able to determine the minimum
subnet size that will support a number of hosts on that
subnet.
 To do this, all you need is to determine the number of
host bits to support the number of hosts and this means
counting in the order of 2

Subnetting
 You should also remember to account for the two (2)
unusable IP addresses in a block which are used for the
network address and broadcast address
 The table below (next slide) shows the number of usable
IP addresses for /31 to /22 (i.e. 1 to 10 host bits)
 To conserve IP addresses, /31 subnets can be used in
cases where there is no need for a network or broadcast
address (e.g. point-to-point links)
 In effect, you can have 2 IP addresses in a /31 subnet if
you use the network and broadcast addresses as host IP
addresses

Subnetting
Host bits per network
No. of Host Bits
Equivalent prefix
length
Subnet Mask Number of usable IP addresses
1 /31 255.255.255.254 21
-2 = 0*
2 /30 255.255.255.252 22
-2 = 2
3 /29 255.255.255.248 23
-2 = 6
4 /28 255.255.255.240 24
-2 = 14
5 /27 255.255.255.224 25
-2 = 30
6 /26 255.255.255.192 26
-2 = 62
7 /25 255.255.255.128 27
-2 = 126
8 /24 255.255.255.0 28
-2 = 254
9 /23 255.255.254.0 29
-2 = 510
10 /22 255.255.252.0 210
-2 = 1022

Subnetting
 You can do the same calculation for other prefix lengths.
 Using this table, we can determine that we need a
minimum subnet size
 of /27 to support 25 hosts
 of /29 to support 4 hosts
 of /25 to support 120 hosts, and so on
Number of Subnets in an Address Block
 Given an address block (network/prefix length), you can
determine the number of subnets that can be gotten from
that address block as long as you know the subnet size
requirements.
 The formula for this is:

 For Example: Calculate the number of /28 subnets from
/24 reference address block
List of Subnets in an Address block
 In the previous example, we determined the number of
subnets that can be gotten from a particular address
block.
 Now, we need to determine what those subnets actually
are.

Sub-netting
 To do this, we need to know the following things:
 The octet in which a subnet exists
1st octet: /1 to /8
2nd octet: /9 to /16
3rd octet: /17 to /24
4th octet: /25 to /32
 The maximum number of bits in the boundary (octet) in
which the subnet belongs
1st octet: 8
2nd octet: 16
3rd octet: 24
4th octet: 32

Sub-netting
 The block size of the subnet
 For example, a /28 subnet exists in the 4th octet. The
maximum number of bits in that octet is 32. Therefore, the
block size is:
 Here’s another example. A /18 subnet exists in the 3rd
octet. The maximum number of bits in that octet is 24.
Therefore, the block size is:
 Example:1
What are the /27 subnets that exist in the 174.53.4.0/24
address block?

Sub-netting
Solution:
 Number of subnets: 227-24 = 23 =8
 The /27 subnet exists in the 4th octet. The maximum number of
bits in that octet is 32. Therefore, the block size is: 232-27 = 25 =32
 Knowing this, we can now list the subnets by starting at first
network of the given block and incrementing by the block
size in the 4th octet:
o 174.53.4.0/27
o 174.53.4.32/27
o 174.53.4.64/27
o 174.53.4.96/27
o 174.53.4.128/27
o 174.53.4.160/27
o 174.53.4.192/27
o 174.53.4.224/27

Sub-netting
 Example:2
List the /23 subnets that exist in the 141.67.128.0/21 address
block.
 Solution:
 the /23 subnet exists in the 3rd octet. The maximum number of bits
in that octet is 24.
 Therefore, the block size is: 224-23 = 21 =2
 Knowing this, we can now list the subnets by starting at first network
of the given block and incrementing by the block size in the 3rd octet:
141.67.128.0/23
141.67.130.0/23
141.67.132.0/23
141.67.134.0/23

Sub-netting
 Example:3
List the /13 subnets that exist in the 131.80.0.0/12
address block.
Solution:
 the /12 subnet exists in the 2nd octet. The maximum number of bits
in that octet is 16.
 Therefore, the block size is: 216-13 = 23 = 8
 The /13 subnets from the 131.80.0.0/12 block are:
131.80.0.0/13
131.88.0.0/13

Sub-netting
 Example:4
What is the valid address range of the 192.168.58.0/28
subnet?
Solution
 The block size is 16 (232-28 = 24 = 16)
 Therefore, the next subnet will be 192.168.58.16/28
(increment the fourth octet by block size)
 As such, the valid address range is:
Start address: 192.168.58.0 + 1 = 192.168.58.1
End address: 192.168.58.16 – 2 = 192.168.58.14
Broadcast address: 192.168.58.16 – 1 = 192.168.58.15

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