Basics about CIDR and VLSM

CIDR

Classless Inter-Domain Routing (CIDR) known as subnet mask or can be used as super netting where, all the IP address and its associated routing prefix is presented. It is noted as “/” such as /0, /10 etc/. It is an appropriate method of designating a subnet mask. It improves the efficiency of the address distribution which was followed in older class based IP address system. It is based on variable length subnet masking (VLSM). 

The earlier classfull address were as below

Class A: over 16 million host identifiers.

Class B: 65535 hosts

Class C: 254 host identifiers.

CIDR Block

A CIDR block is nothing a group of IP addresses that share the same prefix and contain the same number of bits. Super netting occurs when multiple CIDR blocks are connected together to structure a larger network. A prefix’s length determines the size of a CIDR block.

Subnet Mask and their CIDR value

255.0.0.0 : /8

255.128.0.0: /9

255.192.0.0: /10

255.224.0.0: /11

255.240.0.0: /12

255.248.0.0: /13

255.252.0.0: /14

255.254.0.0: /15

255.255.0.0: /16

Similarly, it will continue up to 255.255.255.252 having the CIDR value /30.

For example

If one Class A IP address have default subnet mask is 255.0.0.0, this means the first byte of the subnet mask is all ones (1s). As mentioned below, having CIDR value /8:

11111111.0.0.0 

VLSM

VLSM enables network administrators to divide IP address in to hierarchy of subnets of different sizes. 

For example

If an IP address is 127.10.50.34 and with a subnet mask 255.0.0.0 (8 network bits) then it will be noted as 127.10.50.34/8

There are many sub netting concepts that we can cover later. 

If you have any suggestions, please feel free to post below.

-DR