When we write down IPv4 addresses, no one really has to go over just how you write them. Most people get it after only a little while of seeing it done. 4 sets of numbers, from 0 to 255, each separated by a “.”.
When IPv6 started to be developed, it quickly became apparent that there needed to be an easier way to write it out than (for example)
2001:0db8:0000:0000:0000:0000:0000:0001.
Even if you quickly realize that you could drop the leading zeros (as we naturally do in “dotted decimal”), it’s still fairly unwieldy:
2001:db8:0:0:0:0:0:1
even if it is much better.
So the idea of the double colon was introduced to combine sets of zero’s together. The first RFC’s said that 1 or more groups of 16 bits of zeros could be combined to “::”
2001:db8::1
That’s almost usable! Of course it won’t always go that easily, especially if you start using SLAAC to configure your network:
2001:db8::93e6:baff:febd:6533
Darn. But still better than nothing.
Unfortunately, even with these rules, there wasn’t enough of them for the machines who are the ultimate users of these. By the rules that are defined in earlier RFC’s, all of these are valid:
2001:db8::1
2001:0db8::1
2001:0db8:0::1
2001:0db8:0::0:0001
(etc)
and case wasn’t ever discussed either:
2001:dB8:0::DeAd:BEEF
is perfectly legit.
This makes it harder to write code for things that need to decode these addresses. And RFC 5952 was written to try and address these shortcomings.
Here are the 6 commandments of RFC 5952:
1) Thou shalt not SHOUT your IPv6 address.
- IPv6 must be written in lowercase. 2001:db8::1 not 2001:DB8::1
2) Thou shall destroy leading zeros.
- Always truncate leading zeros. 2001:0db8::1 is not acceptable, you must use 2001:db8::1
3) Thou shalt not use the double colon where there is only one 16 bit set of zeros.
- If you only have one set of 4 zeros, you can no longer use the double colon, instead it just gets shortened to one zero. An address such as 2001:db8:0000:4:5:6:7:8 can’t use the double colon and only gets shortened to 2001:db8:0:4:5:6:7:8
4) Thou shall use the double colon to it’s greatest potential.
- If you have multiple sets of more than 8 zeros, you have to use the set with the most zeros. So if you have 2001:db8:0000:0000:1:0000:0000:0000 you have to use the double colon on the right set of 0’s – 2001:db8:0:0:1::
5) Wheresoever thou has two places to use the double colon, thou shall use the leftmost.
- If there are 2 equal sets of zeros, use the double colon on the one on the left, and single zeros on the right. 2001:db8:0000:0000:1:0000:0000:1 would become 2001:db8::1:0:0:1
6) Thou shall use the square brackets to separate IPv6 address from thy port number.
- When writing an IPv6 address with a port number, use square brackets around the IPv6 address to keep confusion at bay, since ports are appended with a : (the same separator as IPv6 sections): [2001:db8::1]:80 With the square brackets, we know it’s IPv6 address 2001:db8::1 on port 80, not IPv6 address 2001:db8::1:80