MCSA: IPV6 Subnetting Part 2





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Published on Jul 22, 2012

Each global unicast IPv6 address has a 16bit part that is reserved for subnetting. This video looks at how to divide up and allocate this subnet ID in your organization.

There are number of online subnet calculators like subnetonline. These calculators allow you to enter in the IPv6 address and network prefix that you have been allocated and then work out your networks from this. By entering in the number of networks that you will need, making sure to factor in expansion, the tool will work out how many bits you will require. The online tool can also work out the address ranges for each of the networks. In some cases you will have multiple levels. For example, you may want to have routing at the country, state and office level. This tool allows you to define how many subnets that you want at each level and will calculate the subnets for these levels as required.

Simple calculator method
The challenge in a lot of cases when subnetting IPv6 is to work out the increment value. The increment value is the value that is added to your address to represent the start of the next network. In order to do this you first need to work out how many bits you require for the networks that you need. For example if you need 40 networks you would need 6 bits. The largest number 5 bits can hold is 32, 6 bits can hold 64 bits, thus to have 40 networks you need at least 6 bits. The next step is to work out how many of the 16bit subnet masks are not being used. If you were given a network prefix of 50 and you needed 6 bits for subnetting this would give you 56 bits for the network prefix. This would give 64 -- 54 or 10 bits for the subnet masks that are unallocated. When you move from the first network to the second network the network will change from 0 to 1. In binary this would mean the 11 bits of the subnet mask would change from 0 to 1. To work out the increment value, enter in 1 followed by the number of unallocated bits, which in this case would be 10. This would give you the binary value of 10000000000. This would give you a value in hexadecimal of 400. The value of 400 is thus the value that needs to be added to work out which value. For example given the address of 2001:4860:8000/50 the network would be as follows:


Subnetting using math
There are number of formulas that you can use to work out the incremental value for the subnet. In this case I will use the example 2001:4860:8000/56 and add 30 networks to this address.
f = m -- 48
m is the network prefix you have been allocated.
In this example this will give you f = 56 -- 48 which gives a value of 8
Thus f is the number of bits of the subnet ID that have already been allocated andthus cannot be used or subnetted.
i = 2 to the power of (16 -- f -- s)
s is the number of bits that we want to use for subnets. In this case, there will be 30 networks so there will need to be 5 bits.
This gives us I = 2 to the power of (16 -- 8 -- 5) or 2 to power of 3 which is 8
This gives the following addresses

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Reference: IP subnetting

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