Tuesday, 8 May 2012

Difference between VLSM and Subnetting

Hi everyone, One of the most freqeuntly asked question this week is explained below.

Classless Inter-Domain Routing. is also called supernetting

CIDR was first introduced in 1993 by RFC 1517, 1518, 1519, and 1520, and later deployed in 1994.

It's an IP addressing scheme that replaced the older system based on classes A, B, and C. 

The main purpose of CIDR is aggregate route it is also known as supernetting or summarization.when you have to represent three /24

For example you have four C class subnets

we can add 4 routes on the router or we summarize all of them and add a CIDR route whihc is. /22 and covers all the above 4 routes.

CIDR addresses reduce the size of routing tables.

one more way to explain this is CIDR is  prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped into single routing table entries as we have seen above . These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses.


Variable-Length Subnet Masking (VLSM) or Subnetting is used to better utilize address space. Subnets divide a single network into smaller pieces. 

This is done by borrwing bits from the host portion of the address to create a sub network.

Take the class C network The default network mask is, and the last octet contain the host portion of the address. 

To use this address space more efficiently because we dont need all the 254 hosts , we could borrow 3 bits of the last octet for the subnet to make subnets and divide number of hosts equally in each subnetwork.

One point to note is that in subnetting once the subnet mask has been chosen, the number of hosts on each subnet is fixed but you can always further chop it down.I dont wana go in more detail as it is already compex and dont want to make more complex for you. 

Using variable-length subnet masks improves on subnet masking and avoid vaisting . VLSM is similar to traditional fixed-length subnet masking in that it also allows a network to be subdivided into smaller pieces. The major difference between the two is that VLSM allows different subnets to have subnet masks of different lengths. For the example above, a department with 20 servers can be allocated a subnet mask of 27 bits. This allows the subnet to have up to 30 usable hosts on it. 


CIDR = Make one of 6 Network = route summarization
VLSM= Make 6 of 1 Network

This is the best I can explain