Subnetting

Often this comes into the subnetting discussion by my friends who are deploying IPv6 for the first time. How do you calculate subnets outside the 4-bit nibble boundary? This also happens to be one of starting points of APNIC IPv6 routing workshop where I occasionally instruct as community trainer.

 

So what is a Nibble boundary?

In IPv6 context, it refers to 4 bit and any change in multiple of 4 bits is easy to calculate. Here’s how: Let’s say we have a allocation: 2001:db8::/32. Now taking slices from this pool within 4 bit boundry is quite easy. /36 slices (1 x 4 bits) 2001:db8:0000::/36 2001:db8:1000::/36 2001:db8:2000::/36 and so on… /40 slices (2 x 4 bits) 2001:db8:0000::/40 2001:db8:0100::/40 2001:db8:0200::/40 /44 slices (3 x 4 bits) 2001:db8:0000::/44 2001:db8:0010::/44 2001:db8:0020::/44 /48 slices (4 x 4 bits) 2001:db8:0000::/48 2001:db8:0001::/48 2001:db8:0002::/48 Clearly, it seems much simple and that is one of the reasons we often strongly recommend subnetting within the nibble boundary and not outside for all practical use cases. However understanding why it’s easy this way, as well as things like how to subnet outside nibble boundary for cases, say if you are running a very large network and have a /29 allocation from RIR.

One of my friend went for a VM with a German hosting provider. He got single IPv4 (quite common) and a /64 IPv6. Overall /64 per VM/end server used to be ok till few years back but now these days running applications inside LXC containers (OS level virtualization) make more sense. This gives option to maintain separate hosting environment for each application. I personally do that a lot and infect blog which you are reading right now itself is on a LXC container.

Subnetting IPv6 sounds very complex but to be true - it is very easy! All you need to do is to understand basics of IPv6 addressesing - how an address is formed and how to efficiently use CIDR notation.   Firstly how an IPv6 address looks like? (good to clear fundamentals first!) An IPv6 address has 8 sections seprated by coloums and each sections has carries 4 hexadecimal digits. So an IPv6 address is something like: xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx - Each x can have a hexa decimal value i.e from 0 to 9 and a to f. Thus 16 possible values for each x. Since each each x is stored in binary i.e 0 or 1 (that is 2 possible value) - number of bits per section turns out to be 2x2x2x2 = 16bits. Thus we have now each section with 16 bits per section and 8 sections in total. This turns out to be 16 + 16 + 16 + 16 + 16 + 16 + 16 + 16 bits = 128bit. This is why an IPv6 address has 128bits. This means total possible addresses in IPv6 space is 2^128 = 340 282 366 920 938 463 463 374 607 431 768 211 456 addresses. Next, an important point to remember here is  - in IPv6 address clients are mostly based on /64 subnet which means first 64 bits go to network part while next 64 bits go to the host part i.e usage IPv6 addresses which are allocated to end machines.