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Jun 13th, 2007

In cisco curriculum it is written that VLSM is used for hierachical addressing , What do they mean by that

I have this problem too.
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Paolo Bevilacqua Wed, 06/13/2007 - 14:55

Suppose that your ISP assigns you This is also called a /23, or "two class C networks". You then split this space in 4 subnets, each with a mask of /25 /25 /25 /25

These subnets you use for your own, or give to a branch office that in turn can split them again.

This is a "hierarchy" of subnets. You do not need to tell anything to your ISP about this, and the process can repeat upwards and downwards. Another term that is used in relation to this technique, is "aggregation".

Hope this helps, please rate post if it does

mohammedmahmoud Wed, 06/13/2007 - 23:47


Just to add some theoretical description to Paolo's perfect explaination:

Variable Length Subnet Mask (VLSM) and hierarchical addressing:

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.

NOTE: To use VLSM the routing protocol used must support VLSM (classless routing protocol).


Mohammed Mahmoud.


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