IPv6 Subnetting Made Easy


Call it emotional attachment to IPv4 or whatever you like, but recently I have become more and more interested in finding and easy way to subnet IPv6 prefixes in my head like we are used to doing in IPv4. This post expands on an idea I picked off of the NANOG mailing list.

The method presented here is for the quick subnetting in your head, which you should get by practice. So it works well for a small number of subnets (16 max) but can be expanded for any number of subnets. However if you have to deal with more than 16 subnets, it is certainly more efficient to use any of the several IPv6 calculators out there (my favourite is sipcalc – a command line tool).

This method helps you answer the question: “Given the prefix PPPP/Y, what are the sub-prefixes of length /Z (where Z > Y) that can be gotten from it?“

I assume that you understand the fundamentals of IPv6 addressing, very basic binary and hex math. I also assume that you know how to arrive at the value Z from an analysis of your network design or requirements.


  • Hexit – a single hexadecimal digit (0 – 9 , a – e). A hexit is made up of 4 bits.
  • Hex grouping – each of the groups of hexits of an IPv6 address that are separated by colons.
  • Defining hexit – the hexits that change for each subnet but stays the same for the same subnet. Think ‘interesting octet’ from IPv4 subnetting days. You could simply look it up from the following table.

The bit ranges that define various IPv6 hexits

  • Parent prefix – the original prefix to be subnetted in prefix/length format.
  • Y = prefix length of original prefix
  • Z = prefix length of subnets of original prefix

The Procedure

Calculate the following parameters

  1. Number of subnetting bits:  x = Z – Y

  2. Number of subnets possible with x bits: S = 2^x

  3. Number of hexits represented by the x bits: h = x/4 (this should be rounded up to the next integer)

For a given number of subnet bits (x), the defining hexit for each subnet will increase in value according to the increment, from hexadecimal zero (0) to “e” as shown in the table below. Note that the ‘definining’ hexit(s) is simply the one after the boundary hexit (that defined by original prefix length Y.

IPv6 Quick Subnetting Cheatsheet

IPv6 Quick Subnetting Cheatsheet


[a] Subnet 2001:db8::/32 into /36s

  • Subnetting bits (x) = 36 – 32 = 4 (these will give us 16 [i.e. 2^4] subnets.

  • Number of hexits represented by 4 bits = 4/4 = 1

  • From the table, the defining hexit should increment by 1 with discrete hex values 0 – f.

  • Thus our subnets are:


[b] As a small end site, you received the 2001:db8:c001:ba00::/56 assignment from your ISP, and you have 3 main office locations you want to divide this block between. Find these 3 subnets.

  • Number of subnets required = 3

  • Subnetting bits (x) such that (2^x) >=3 , x = 2 (this will actually give us 4 subnets of equal prefix length). Also the prefix length of our subnets will be 58 (56+2)

  • Number of hexits represented by 2 bits = 1

  • From the table, the defining hexit should increment by 4 with discrete hex values 0, 4, 8, c.

  • Thus our subnets are:

Please drop me a comment if you have suggestions of how to make this even easier and happy v6 subnetting …in your head. If you would like a highly detailed and complete method for IPv6 subnetting, let me invite you to read my other post “IPv6 Subnetting – General Procedure

Ubuntu Idyll Cisco Study Environment

I have written a few posts in which I mentioned the amazing GNS3/Dynamips/Dynagen set of tools that enable you emulate a Cisco network …. also a great too used to prepare for the Cisco CCIE lab exam [Professor of Internetworking]

Here are a couple of things I have used that make creating and using labs a nirvana experience in Ubuntu … [constant flow for those who understand Emotional Intelligence]

  1. Because I hate having multiple console windows open, one for each router which is the default behaviour of GNS3, I wrote the script [tamonet.sh] in the previous post that essentially takes as an arguement your topology (.net) file and then opens each router console in a separate tab in gnome-terminal.
  2. I modified my gnome-terminal profile so that my background is translucent. That way, I can have a document (e.g a workbook) I refer to in the background and read instructions or make references without having to switch windows.
  3. Initially, to launch my lab with my tamonet.sh script, I would open up either a terminal or the RUN application box [Alt-F2] and issue the command …. that is until I discovered awn-terminal. Boy! …. first of all, AWN is a cool dock application for Ubuntu and one of its applets is a nifty cool terminal that you just click and it pops up a next-gen-like translucent terminal. You just type in your command and as soon as u move focus from it, it automatically closes … no clutter on your desktop!!
  4. And finally … when I am doing such work, I play music. Mozart is ideal for me but I generally prefer any kind of music without vocals. It is the vocals I find distracting. So for me, to complete the ensemble, it is Mozart, Beethoven, Tchaikovsky or soundtrack music [Prison Break, Stargate etc]

Given what I mostly do these days [putting in my 10000 hours to master my craft as a network engineer] and my passion for Ubuntu … I enjoy every single moment I spend building and executing scenarios and everyday, I say thank you to the guys who selflessly make all this possible …and it reminds me to keep sharing what little I know. Happy Valentines day guys and babes … do spend vals with someone real … far away from your laptop ok?