How to subnet an IPv6 network – a 48 block example

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Added by Mark Jacob August 2, 2021

An IPv6 subnetting example:

You are issued a /48 block from your ISP. Subnet it so you have more than 500 networks.

Red numbers are the original 48 bits of the network issued by your ISP. The red bits do not change in this example.

Must borrow 9 more bits (2⁹ > 500) to get more than 500 networks.
Blue numbers are newly borrowed network bits.

Green numbers are host bits. Counting the remaining 64 host bits, plus the 7 bits in hex values 15 & 16, there are 71 host bits in total.

71 host bits means each of the 500 networks that have been created will support 2⁷¹ hosts, or 2.36 x 10²¹ hosts.

Note that all IOS entries to test the IPv6 addresses in this spreadsheet were on a 3640 router running IOS 12.4.

Download this PDF single instance of an example

There are plenty of sites that allow you to input information and see the resulting subnet IDs. I wanted to provide an example where you could actually see what happens to the individual bits. Hit me with any comments,

Mark Jacob