Variable Length Subnet Mask
A Variable Length Subnet Mask (VLSM) is a numerical masking sequence, or IP address subset,
based on overall network requirements. A VLSM allows a network administrator to use long
masks for networks with few hosts and short masks for networks with multiple hosts.VLSM
enables you to have more than one mask for a given class of address, albeit a class A, B, or C
network number.
VLSM, originally defined in RFC 1812, allows you to apply different subnet masks to the same
class address space Classful protocols, such as RIPv1 and IGRP, do not support VLSM. To deploy
VLSM requires a routing protocol that is classless—BGP, EIGRP, IS-IS, OSPF, or RIPv2, for
instance.
VLSM provides Two major advantages:
more efficient use of addressing
Ability to perform route summarizationwhen you perform classful subnetting, all subnets have the same number of hosts because they all
use the same subnet mask. This leads to inefficiencies. For example, if you borrow 4 bits on a
Class C network, you end up with 14 valid subnets of 14 valid hosts. A serial link to another router
only needs 2 hosts, but with classical subnetting, you end up wasting 12 of those hosts. Even with
the ability to use NAT and private addresses, where you should never run out of addresses in a
network design, you still want to ensure that the IP plan that you create is as efficient as possible.
An efficient addressing scheme using VLSM.
- Find the largest segment in the area—the segment with the largest number of devices
connected to it. - Find the appropriate subnet mask for the largest network segment.
- Write down your subnet numbers to fit your subnet mask.
- For your smaller segments, take one of these newly created subnets and apply a different,
more appropriate, subnet mask to it. - Write down your newly subnetted subnets.
For even smaller segments, go back to step 4.
Route Summarization
Route summarization is the ability to take a bunch of contiguous network numbers in your
routing table and advertise these contiguous routes as a single summarized route.
Route summarization, or supernetting, is needed to reduce the number of routes that a router
advertises to its neighbor. Remember that for every route you advertise, the size of your update