This article is the third in a series of three which has been written for those who are, or may one day be responsible for designing an IP network.

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Sit back, take a deep breath and prepare for another of my techie articles, this time on how to configure Variable Length Subnet Masking (VLSM).

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VLSM’s are used to reduce the number of wanted IP Addresses in each subnet, allow for more subnets and avoid having to obtain another registered IP Address.Again the best way I have found to explain VLSM’s is to run through an example configuration

Example

Your company’s network has been assigned a Class A registered address of: 98.0.0.0 from this you need to create:

• Six subnets of 2000000 Hosts
• Thirty subnets of 64000 Hosts
• Fourteen subnets of 4000 Hosts

The default Class A subnet mask is 255.0.0.0, represented as / 8.

The best way to approach this is to work in stages calculating the different subnet requirements

Step 1: - Six subnets of 2000000 Hosts

The original network address is: 98.0.0.0 / 8

The number of subnets needed is 6 - to achieve this 3 bits must be borrowed from the host portion of the IP Address (second octet) as shown below.

This will leave 21 bits for the hosts (5 bits from the second octet plus all of the third and forth octets). This will give 2097150 host addresses

The new network address would therefore be 98.0.0.0 / 11 (the original 8 + 3 bits)

Each subnet will have a different network address in increments of 32 for the second octet, therefore the second subnet would have the address: 98.32.0.0 /11. The seventh unused octet would have the address 98.192.0.0 /11

Step 2: - Thirty subnets of 64000 Hosts

The new network address is 98.192.0.0 / 11

The number of subnets needed is 30 - to achieve this, 5 bits must be borrowed from the host portion of the IP Address (third octet).

Leaving 16 bits (All of the third and forth octets) for the hosts giving 65534 host addresses

The new network address would therefore be: 98.192.0.0 / 16 (the original 11 + 5 bits)

Each subnet will have a different network address in increments of 1 for the second octet, therefore the second subnet would have the address: 98.193.0.0 / 16

Step 3: - Fourteen subnets of 4000 Hosts

The new network address is 98.255.0.0 / 16

The number of subnets needed is 14 - to achieve this 4 bits must be borrowed from the host portion of the IP Address (third octet)

This will leave 12 bits (4 bits from the third octet plus all of the forth octet) for the hosts giving 4094 host addresses

The new network address would therefore be 98.255.0.0 / 20 (16 + 4 bits)

Each subnet will have a different network address in increments of 16 for the third octet, therefore the second subnet would have the address:: 98.255.16.0 / 20.

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So thats it, you should now understand variable length subnet masks – Until the next time - Good Luck

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