What is the difference between CIDR and VLSM?

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Jan 9th, 2009

What is the difference between CIDR and VLSM ?

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Roberto Salazar Fri, 01/09/2009 - 20:09

In CIDR , an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. Length means the number of left-most contiguous mask bits that are set to one. So network can be represented as CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network, then the ISP can offer,, and so on to customers. Yet, when advertising to other providers, the ISP only needs to advertise

Variable Length Subnet Masks (VLSM) allows you to use different masks for each subnet, thereby using address space efficiently.

VLSM Example

Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:

netA: must support 14 hosts

netB: must support 28 hosts

netC: must support 2 hosts

netD: must support 7 hosts

netE: must support 28 host

so the difference is CIDR is supernetting while VLSM is explained better with the following: can be broken down into 5 different networks given the example requirement above into:

netB: host address range 1 to 30

netE: host address range 33 to 62

netA: host address range 65 to 78

netD: host address range 81 to 94

netC: host address range 97 to 98

Clear as mud?

Giuseppe Larosa Sat, 01/10/2009 - 02:43

Hello Badalam,

both move futher from Class based networking (Class A,B,C) with different targets.

Classless InterDomain Routing allows to perform supernetting:

for example you need to advertise (a Class C)

to (another Class C)

Before CIDR creating a single aggregate of 16 Class C major networks was not allowed now it is so a service provider that owns all this block can advertise in BGP

This helps in reducing the size of full Internet BGP table (however, they are always growing for other reasons)

VLSM= Variable Length Subnet Mask

the context is interior routing within an enterprise.

By using a routing protocol that carries the subnet mask is possible to use different subnet masks as needed:

/30 on point-to-point serial links

/28 - /25 on Lan segments

so CIDR moves to the left the network boundary behind Class A,B,C specifications, VLSM allows to move network boundary to the right providing felxibility in the usage of subnets of major network(s)

Hope to help


scottmac Sat, 01/10/2009 - 07:07

Adding to the previous posts, I believe you'll find that CIDR is a concept applied more at the ISP/Provider level, were VLSM is a mechanism used by the recipient of an address block to use the assigned address block more efficiently.

If the provider gives you, as a user, a /24 block (from any address range, a CIDR block), you will likely use them by subnetting them into a smaller blocks. If you choose to, you can address the smaller blocks such that the various subnets are not all equal in size.

To the internal router / routers that are handling those various-sized subnets, you must apply the appropriate (V)ariable (L)ength (S)ubnet (Mask) ... multiple subnets of varying host counts per subnet, independent of each other, and non-overlapping in their addressing structure.

CIDR is the chunk of meat the provider gives you, VLSM is how you serve it to your guests (some get a lot, some get a little).

Good Luck



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