Easy Understanding Variable Length Subnet Mask - VLSM

 

Variable Length Subnet Mask (VLSM)

When you apply same subnet mask for all subnets in the network, the range of host addresses for each subnet will be same. If your requirement for hosts in each subnet varies, the address space in the subnets is wasted. To use the addresses space efficiently, you can use the Variable Length Subnet Mask (VLSM). It allows you to use different masks for each subnet.

If a Cisco router having a subnet mask of certain value uses the routing protocol RIP, it assigns that subnet mask in its address space to all the interfaces. This is referred as classful routing. The classful protocols such as Routing Information Protocol version 1 (RIPv1) and IGRP do not support VLSM. The classless protocols such as Enhanced Interior Gateway Routing Protocol (EIGRP), open Shortest Path First (OSPF) or RIP version 2 (RIPv2) allow you to deploy VLSM. There are two advantages of VLSM given as follows:

Efficient use of IP addressing

Route summarization

Addressing with VSLM

You can implement VLSM to apply subnet masks of different lengths to a network for the purpose of subnetting. One of the major advantages of VLSM is to allow more efficient use of IP addressing. You can create subnets from the subnetted subnets. This can be accomplished by performing following steps:

1. Find the segment in the network to which majority of the devices is connected.

2. Calculate the subnet mask for that segments.

3. List the subnet numbers to which you can apply the calculated subnet mask.

4. Apply a different subnet mask to the newly created but smaller network segments.

5. List all the new subnets to which subnetting is applied.

6. Apply appropriate subnet masks for even more small subnets.

Route Summarization

Route summarization is taking the adjacent network numbers in a routing table and representing them as the smaller number of routes. VLSM allows you to summarize the subnetted networks as a single route. For example, if a network with IP address as 172.160.1.0/24 is divided into four subnets with IP addresses 172160.1.0/26  you can summarize the subnets as the network number 172.160.1-0/24 in the routing table.

The following are the advantages of using route summarization:

Reduction in the size of routing table, memory requirement and time for processing.

Reduction in the size of updates and bandwidth requirement.

Detection of networking problems that ensures proper routing of the packets to the destination.

VLSM Design

A VLSM design ensures efficient use of available IP addresses as well as more-efficient routing update communication using hierarchical IP addressing. While developing VLSM design for a network, you must consider the following design criteria that affect the functioning of the VLSM technology:

Total subnets required currently.

Total subnets that may be required in the future.

Number hosts on the largest subnet currently.

Number of hosts that may be required on the largest subnet in future.

Considering these factors, you can implement the VLSM scheme by creating VLSM masks. You require block sizes to create the VLSM mask. lists the chart of block sizes that can be used for creating the VLSM mask for Class networks.

Subnet	Masks	Subnets	Hosts	Block Size
/25	128	-	126	128
/26	192	2	62	64
/27	224	6	30	32
/28	240	14	14	16
/29	248	30	6	8
/30	252	62	4	4