IP Addressing

IP address - A unique identifier that designates the location of a device on a network and it's membership to that network.

IP Facts

  • An IP address is 32 bits long
  • IP addresses are usually expressed in dotted-decimal format
  • Each IP address consists of 4 octets
  • IP addresses are structured hierarchical addresses
  • IP addresses are split into network and host parts

IP addresses are hierarchical by design, it allows routers to determine routing decisions much more efficiently since the router only needs to know what network the destination address belongs to in order to route the packet.

If IP addresses were flat addresses every router would need to know the paths to reach every host. Considering that there are 4.2 billion possible IP addresses this would be hard to manage.

The network and node portions of an IP address are determined by the class of the IP address you have been assigned. There are three main classes of IP address. Class A, Class B and Class C.

Class A addresses use one octet for the network portion and three octets for the host portion. Consequently Class A addresses can have the most hosts: namely 16,777,214 hosts for each class A address.

Class B addresses use two octets for the network portion and two octets for the host portion. Class B networks can have 65,534 hosts per network.

Finally Class C addresses use three octets for the network portion and one octet for the host portion, leaving Class C addresses only capable of supporting 254 hosts.

To calculate the number of hosts for a given network use the following equation.

2n - 2 = number of hosts

n = number of host or node bits

Every network or subnetwork has two IP addresses that should not be assigned to a host. They are called the "Network" or "Wire" address and the "Broadcast" address. These IP addresses are the first and last IP addresses in the network or subnetwork range respectively.

This is the reason why the equation above deducts 2 from the result.

For example Suppose we look at the IP address This is a class B IP address. Since class B IP addresses use two octets for the network portion we can safely assume that for this unsubnetted IP address the network address is (All zeros in the host portion) and the subnet mask would be

Class A - network.node.node.node

Class B - network.network.node.node

Class C - network.network.network.node

There is an easy way to determine the class of an IP address. Convert the first octet of the IP address into binary, the leading bit pattern of the first 2 bits will determine the class.


Example: The table shows the first octet of in binary.

ClassLeading Bit Pattern

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