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.

Going back to fundamentals

IPv6 address consists of 128 bit addressing and is represented in hexadecimal.
IPv6 address:  _ _ _ _: _ _ _ _ :_ _ _ _ :_ _ _ _ :_ _ _ _ :_ _ _ _ :_ _ _ _ :_ _ _ _ 
Each dash here represents is written in hexadecimal and represents 4 bits, thus 4+4+4+4 = 16 bits in each block and 16 x 8 = 128 bit addressing. This brings that magical 4-bit nibble boundary.
So if we expand 4 bits into binary, we can have following combinations for each “dash” in above representation:

0 0 0 0 
0 0 0 1
0 0 1 0
0 0 1 1 
0 1 0 0 
0 1 0 1
0 1 1 0
0 1 1 1 
1 0 0 0 
1 0 0 1 
1 0 1 0
1 0 1 1 
1 1 0 0 
1 1 0 1
1 1 1 0
1 1 1 1

Here I have simply represented 4 bits from lowest to highest. Remember just like in the decimal system with base 10 (which we all are familiar with), we follow same logic in binary system where we start from lowest (0 0 0 0) and go to next digital (0 0 0 1) and now since it’s base 2, we go to next logical number which is (0 0 1 0) and so on. Now when we modify these 4 bits together, we do not have to worry about the decimal part but as soon as we try to go inside the 4-bit zone, we have to deal with the decimal counting.
So let’s take a real-world case of American Cable & broadband provider Comcast. They have an allocation 2001:558::/31:

NetRange: 2001:558:: - 2001:559:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF
CIDR: 2001:558::/31
NetName: COMCAST6NET
NetHandle: NET6-2001-558-1
Parent: ARIN-001 (NET6-2001-400-0)
NetType: Direct Allocation
OriginAS: AS7922
Organization: Comcast Cable Communications, LLC (CCCS)
RegDate: 2003-01-06
Updated: 2016-08-31
Ref: https://whois.arin.net/rest/net/NET6-2001-558-1

What exactly /31 means here?

Going back by CIDR fundamentals /31 means 31 bits are reserved and remaining (128-31 i.e 97 bits) are available. How can they generate /32 or say /36 out of this allocation?
2001:558::/31
Writing in expanded form:
2001:0558::/31
(16 bits + 15 bits)
In above, first 16 bits are reserved for 2001 but for next part “0558” only 15 bits are reserved. Let’s expand the 2nd block further:
0 5 5 8 – 15 bits reserved
Here “0” gives 4 bits (and in binary is 0 0 0 0)
5 gives 4 bits (and in binary is 0 1 0 1)
Next 5 also reserves 4 bits
So far we are at (4 + 4 + 4) 12 bit count. Now that 15 bits are reserved, basically from “8” 3 bits are reserved and rest 1 bit is available for modification.
Let’s expand 8:
8 in hexadecimal = 1 0 0 0 in binary. Here 1 0 0  is reserved (each representing one binary bit and hence the three bits) and 4th bit can vary.
Hence possible combinations in binary are:
1 0 0 0
1 0 0 1
The remaining first three bits (1 0 0 ) cannot be altered as they are part of network mask. Now 1 0 0 0 in binary gives us “8” in hexadecimal and 1 0 0 1 gives us “9”. Thus possible /32s out of this /31 allocation are:
2001:558::/31 = 2001:558::/32  and 2001:559::/32
Similarly to calculate /36 slices from it, we can basically vary this 1 bit (as we just did) as well as next 4 bits altogether (5-bit variation). Hence possible /36 slices are:
2001:558:0000::/36
2001:558:1000::/36
2001:558:2000::/36
2001:558:3000::/36
and so on until 2001:558:f000::/36 (16 pools here)
and next,
2001:559:0000::/36
2001:559:1000::/36
2001:559:2000::/36
2001:559:3000::/36
and so on until 2001:559:f000::/36 (16 pools here). Thus we get these 32 /36 blocks out of /31 allocations.
That’s all about IPv6 subnetting. Once you understand this part, you should be just fine with subnetting in the future. 🙂