Unanswered Question
Richard Burts Fri, 06/01/2007 - 12:15


For many purposes we can consider CIDR and VLSM as basically interchangeable terms. They both involve changing our concept of how we do IP addressing, subnetting, and routing to move from a classful outlook to a classless outlook.

If you really want to understand the difference, then CIDR is related to how we make routing decisions and VLSM is how we assign and manage subnets.



mohammedmahmoud Fri, 06/01/2007 - 12:31


Rick has explained it perfectly, but i would like to say it from another perspective:

Variable Length Subnet Mask (VLSM), is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule, in order for good utilization of the address space. With VLSM, a network administrator can use a long mask on networks with few hosts and a short mask on subnets with many hosts.

CIDR (Classless Inter-Domain Routing ) on the other hand is the action of summarization (Supernetting) at the network boundaries, in order to minimize the routing updates and thus reduce the size of the routing tables, reducing the convergence time and reducing the utilization of the routers resources also.

To use both CIDR and VLSM the routing protocol used must support them (classless routing protocol).

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Richard Burts Fri, 06/01/2007 - 12:50


Another perspective is frequently useful. I welcome your approach to explanations which may help Neo to understand it better.



mohammedmahmoud Fri, 06/01/2007 - 12:58

Hi Rick,

Thank you for your professional attitude, we are all here to interact for the best of us all, i am really glade to be part of the forum, side to side with experts like your self.


Mohammed Mahmoud.


This Discussion