IP Addressing: Classless Addressing | Computer Networks - Computer Science Engineering (CSE) PDF Download

Network Address and Mask

• Network address: It identifies a network on internet.  Using this, we can find range of addresses in the network and total possible number of hosts in the network.
• Mask: It is a 32-bit binary number that gives the network address in the address block when AND operation is bitwise applied on the mask and any IP address of the block.

The default mask in different classes are 

• Class A – 255.0.0.0
• Class B – 255.255.0.0
• Class C – 255.255.255.0

Example: Given IP address 132.6.17.85 and default class B mask, find the beginning address (network address).

The default mask is 255.255.0.0, which means that the only the first 2 bytes are preserved and the other 2 bytes are set to 0. Therefore, the network address is 132.6.0.0.

Subnetting

Classless Addressing

Some values calculated in subnetting

• Number of subnets: 2^{(Given bits for mask – No. of bits in default mask)}
• Subnet address: AND result of subnet mask and the given IP address
• Broadcast address: By putting the host bits as 1 and retaining the network bits as in the IP address
• Number of hosts per subnet: 2^{(32 – Given bits for mask)} – 2
• First Host ID: Subnet address + 1 (adding one to the binary representation of the subnet address)
• Last Host ID: Subnet address + Number of Hosts

Example: Given IP Address – 172.16.0.0/25, find the number of subnets and the number of hosts per subnet. Also, for the first subnet block, find the subnet address, first host ID, last host ID and broadcast address.

This is a class B address. So, no. of subnets = 2^(25-16) = 2⁹ = 512.

No. of hosts per subnet = 2(^32-25) – 2 = 2⁷ – 2 = 128 – 2 = 126

For the first subnet block, we have subnet address = 0.0, first host id = 0.1, last host id = 0.126 and broadcast address = 0.127