VLSM and Route Summarization

VLSM

VLSM occurs when an internetwork uses more than one mask in different subnets of a single Class A, B, or C network.  VLSM allows engineers to reduce the number of wasted IP addresses in each subnet, allowing more subnets and avoiding having to obtain another registered IP network number from regional IP address assignment authorities.

At this stage, I actually took a little bit of time to re-remember subnetting.  Had to brush back up on it.

A common VLSM mistake is thinking “using more than one mask” rather than “using more than one mask in a single classful network”.

Classless and Classful Routing Protocols

For a routing protocol to support VLSM, it must advertise not only the subnet number but also the subnet mask when advertising routes.  A routing protocol must include subnet masks in its routing updates to support manual route summarization.

Each IP routing protocol is considered to be either classless or classful, based on whether the routing protocol does (classless) or does not (classful) send the mask in routing updates.  All routing protocols are either classless or classful.

Routing Protocol	Is It Classless?	Sends Mask in Updates	Supports VLSM	Supports Manual Route Summarization
RIP-1	No	No	No	No
IGRP	No	No	No	No
RIP-2	Yes	Yes	Yes	Yes
EIGRP	Yes	Yes	Yes	Yes
OSPF	Yes	Yes	Yes	Yes