IPv6 Subnetting Made Easy

Introduction


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.


Definitions


  • 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

Examples


[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:

2001:db8:0000::/36
2001:db8:1000::/36
2001:db8:2000::/36
2001:db8:3000::/36
2001:db8:4000::/36
2001:db8:5000::/36
2001:db8:6000::/36
2001:db8:7000::/36
2001:db8:8000::/36
2001:db8:9000::/36
2001:db8:a000::/36
2001:db8:b000::/36
2001:db8:c000::/36
2001:db8:d000::/36
2001:db8:e000::/36
2001:db8:f000::/36

[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:

2001:db8:c001:ba00::/58
2001:db8:c001:ba40::/58
2001:db8:c001:ba80::/58
2001:db8:c001:bac0::/58
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
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53 thoughts on “IPv6 Subnetting Made Easy

  1. amazing Good Article ,the people just like u hard things can learn easy too.Great man,i also confuse regarding about IPv6 now i can get some of idea about this appreciate ,well-done.

    • The quickest way to uesardtnnd subnetting is that subnetting is a binary AND operation of all the octets in your IP address with the subnet mask. subnet binary 255 (11111111) always allows the octet to pass thru and becomes the same number in the network address. performing an AND operation if the mask octet is less that 255 will yield a consistent number that will become the lowest number of the network address octet. A smaller mask octet number will yield a wider network range for the chosen IP address, and this number range is 256 minus the mask octet. For example, a mask of 255.255.254.0 will yield a 2 x 255 address range for the chosen IP address. Lowest number in the range is the network address and the highest is the broadcast address.

  2. Your example A suffers from incorrect syntax. The significant character in a /36 address is the 9th character, so the correct addresses for your /36 subnets within the specified /32 are:

    2001:0db8:0000::/36
    2001:0db8:1000::/36
    2001:0db8:2000::/36
    2001:0db8:3000::/36

    …and so forth. I suspect you know that; however, you compressed the example incorrectly. You MUST NOT use the double colon to remove trailing zeroes from within a 4-character section; the double colon is only to summarize entire sections of zeroes. If there are fewer than 4 characters in a section, the missing characters are always leading zeroes, not trailing zeroes, even if followed by a double colon.

    So the numbers you posted in your example:

    2001:db8:0::/36
    2001:db8:1::/36
    2001:db8:2::/36
    2001:db8:3::/36

    …actually mean something else entirely:

    2001:0db8:0000::/36
    2001:0db8:0001::/36
    2001:0db8:0002::/36
    2001:0db8:0003::/36

    …which is nonsense; those are different /48 ranges all within the same /36.

    • You certainly can but I just don’t see the point, for more complicated stuff, i’d just use sipcalc or some other tool. However as an academic exercise, i will extend the examples to larger number of subnets. ;-)

      • Hi,I got some idea about Cisco ipsec vpn from your posts. But I still can not find the answer. I need craete an ipsec tunnel between home and warehouse. Juniper SSG5 is put at home and a cisco ASA is put at warehouse.I build a remote access tunnel at ASA side. It is working fine if access server at warehouse from home.But I also need to access a server at home from warehouse. It can not using cisco remote access vpn. I can not use cisco site to site vpn because home side has no static ip. Can you please give me any suggestions?Thanks,Yan

  3. Mukom is a great teacher, i attended your class this week in Gambia, but the way you explain it here is much easier to understand than in class, or my i like to read things at my private space. but really good you did. Mohamed Faye Qcell

    • Thanks Mohammed … am beginning to think I should just give the short version in class and refer everyone to this blog for details!

  4. Great example! Well done. One thing I would add would be a calculation ot the increments. I would rather have a way of calculating them than remember a chart. the values for “x” and the number of subnets can be caqlculated. Your example use a the chart to return the descrete values for the defining hexit. If you take 16 and divide it by the number of subnets you get an increment value that looks (to me) much like the “magic number” in an ipv4 calculation. For example if I was building 4 subnets as in example [b] my increment would be 16/4=4, and my discrete values would be 0,4,8,c(12). I realize that I am coming up with the same values but the less I have to memorize the happier I am. Again GREAT example! Well done!
    Michael Schindler

  5. Great explanation.

    One question
    In your /32’s to /36’s example you’ve left out, 2001:db8:9000::/36 which I assume is a oversight.

    But you’ve also left out
    2001:db8:f000::/36

    Is there a reason for this?

    • Ooops (I did it again) … the :9000 and also :f000 were omissions. There should be 16 subnets i.e. 2^4. Thanks for pointing them out … and thank God for readers like you show keep me honest.

    • I dont understand “Number of hexits represented by 2 bits = 1 and increment by 4″ compared to “Number of hexits represented by 4 bits = 4/4 = 1 and increment by 1″. Both equals 1 but once is incremented by 4 while the other is incremented by 1

  6. Appreciate your fast reply but I still dont get it.
    In the first example: 4/4 = 1 which makes the incrmeent of 1
    In the second example 2/4 = 1 but the increment is 4?
    Why in example 1 and 2 both resulting in 1 but the increment in different? I am kinda slow, sorry if raking your time.

  7. Could you explain about the table the one that has “The bit ranges that define various IPv6 hexits” underneath it. I am trying to understand
    1. The area where the box is colored green , brown , yellow and blue?
    2. how you derived the number 1,5,9 and 13?

    • (1) Each colour band represents a group of 4 hexits (or 16 hexits total). Notice that each is separated by a colon “:”
      (2) Knowing that each hexit is four bits, the first hexit is starts from bit 1 – 4, the next hexit is represented by bits 5 – 9 etc.
      I hope this helps.

  8. See http://www.sput.nl/internet/cidr-routing.html and scroll to the bottom you’ll find table for /64 /60 /56

    If you look at the Fourth Hexit it is the key to subnet address range each subnet being a successive up to 64 filled bits on the right with an increment in the fourth hexit to the next subnet range.
    /64 is only one subnet 0000::: to 1111:1111:1111:1111
    /60 is fff0 to ffff = 16 subnets see 4th hexit ie fff1 fff2 fff3 fff4 fff5 to ffff
    /56 is ff00 to ffff = 256 subnets see 4th hexit is ff01 .. ffc5 …ffd6 …. ffff

  9. Sorry in last comment I meant /64 is 0000::: to ffff:ffff:ffff:fff, as in all binary 1s filled in right most 4 hexits with fourth hexit ffff — very similar to a 255.255.255.0 IPV4 mask

  10. Very good way to explain this. However, I do not know if we can call a “subnet” something with a prefix < /64. One /36 for example is not a subnet in IPv6. It is a group of 2^28 /64s.

    • Hi Carlos, I use ‘subnet’ here in the context of part of a network. If you’d rather you could also call it sub-prefix. Taking your example while a /36 is a supernet of several longer prefixes, it is also a subnet of a /32 for example. Network is not only /64.

  11. Another Cheat Sheet for doiing IPV4 also: – http://www.aelius.com/njh/subnet_sheet.html
    I think the tables are the easiest way. Also if you want to do the math you can use the programmer view on your computer’s calculator.It has hex,decimal and binary with logical operators. Loved the algebra in the examples, I checked and gave me Network Id when ANDED so should work.

    • In the /58 Example you could I think maybe use the same figures to nest a parent subnet as in
      2001:db8:c001:ba00::/58 t
      2001:db8:c001:ba40::/58
      2001:db8:c001:ba80::/58
      2001:db8:c001:bac0::/58
      If you put 0 4 8 and C behind 0 4 8 C you get 4 subnets with 4 subnets to each one
      2001:db8:c001:b000::/58
      2001:db8:c001:b440::/58
      2001:db8:c001:b880::/58
      2001:db8:c001:bcc0::/58
      I think you can do that ?

      • In previous comment no you cant do that as it is a provided fixed Global Id which you cant nibble at on that side. You can on the other side though but youd have to move your mask along another C – 1100 or F – 1111 to cut to the network id for routing.etc

  12. Does anyone know if its OK if you want keep your IPV4 schema can you use the last 32 bits and use your IPV4 addresses converted to Hex in your new IPV6 schema. Is IPV6 limited only to subnetting within the fourth Hexet ? Is VLSM allowed ? It just seems like too many host ids for efficient hopping and routing using addresses past the fourth hexet so why not use the last 32 bits with hex conveted IPV4 addresses.It would make migrating IPV4 to IPV6 so easy if you could…

    • Yes that is possible. So if your webserver is 192.0.24.28, you can have your IPv6 address for the same web server as 2001:db8::192.0.24.28. Note that in this case the 192 in the v6 address is not equal to the 192 in the v4 address but once you do this, you can simply look at the v6 address and correlate it to your IPv6 space. Of course you can only this when you are manually configuring addresses. I am not aware of any hacks to make that work in SLAAC or DHCPv6

      • Maybe Microsoft should create a DHCPv6 service for it to encourage using IPV6 from IPV4. Surely its much more efficient than using the fouth hexet and having thousands of unused host-ids to hop along. I

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  18. Here’s a question: Why subnet with IPv6? Aren’t the chances of duplicate addresses so astronomical that we can just use DHCP?

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