Variable Length Subnet Mask Slides, Cheat Sheet of Computer Networks

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VLSM

Variable Length Subnet Mask

VLSM

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.

Variable-length subnet masking (VLSM) is the process of “subnetting a

subnet” and using different subnet masks for different networks in your IP

plan. What you have to remember is that you need to make sure that there is

no overlap in any of the addresses.

The steps to create an IP plan using VLSM for the network as follows: Step 1 Determine how many H bits will be needed to satisfy the largest network. Step 2 Pick a subnet for the largest network to use. Step 3 Pick the next largest network to work with. Step 4 Pick the third largest network to work with. Step 5 Determine network numbers for serial links.

Step 1 Determine How Many H Bits Will Be Needed to Satisfy the Largest Network

A is the largest network with 50 hosts. If 2 H

• 2 = Number of valid hosts per subnet Then 2 H
• 2 ≥ 50 Therefore H = 6 (6 is the smallest valid value for H) Now have: NNHHHHHH

Two borrowed H bits means a subnet mask of 11111111.11111111.11111111. or 255.255.255. or / Pick one of these subnets to use for Network A. The rest of the networks will have to use the other three subnets.

Step 3 Pick the Next Largest Network to Work With Network B = 27 hosts Determine the number of H bits needed for this network: 2 H

• 2 ≥ 27 H = 5 Pick one of the remaining /26 networks to work with Network B. For the purposes of this example, select the .128/26 network: 10000000 But you need only 5 H bits, not 6. Therefore, you are left with 10N

The original subnet mask of /24 was changed into /26 for Network A. You then take

one of these /26 networks and break it into two /27 networks:

10000000 and 10100000 both have 3 N bits and 5 H bits. The mask now equals: 11111111.11111111.11111111. or 255.255.255. or / Pick one of these new sub-subnets for Network B: 10000000 /27 = Network B

make sure the addresses are not overlapping with each other. So go back to the

original table.

These new sub-subnets will now have new subnet masks.

Each sub-subnet now has 4 N bits and 4 H bits, so their new masks will be

11111111.11111111.11111111. or 255.255.255. or /

Because you have 4 N bits, you can create 16 sub-subnets from the original subnet:

You need only four of them. You can hold the rest for future expansion or

recombine them for a new, larger subnet:

All these can be recombined into the following:

Going back to the original table, you now have the following:

Looking at the plan, you can see that no number is used twice. You have now created an IP plan for the network and have made the plan as efficient as possible, wasting no addresses in the serial links and leaving room for future growth. This is the power of VLSM!

A router that needs to advertise ten routes needs ten specific lines in its update packet. The more routes you have to advertise, the bigger the packet. The bigger the packet, the more bandwidth the update takes, reducing the bandwidth available to transfer data.

Four-City Network Without Route Summarization