IPv6 addresses: format and shortening rules

Why IPv6?

IPv4 has about 4.3 billion addresses, and the free pool ran out years ago. Private addresses and NAT have stretched it, but NAT adds complexity and breaks some applications. IPv6 (Internet Protocol version 6) solves the shortage with 128-bit addresses: 2^128, about 3.4 × 10^38 addresses, enough to give every device its own global address for a very long time.

India is one of the big IPv6 users in the world. Large mobile operators, such as Reliance Jio, built their networks with IPv6 at the core, so a large share of mobile traffic in India runs over IPv6, and India appears near the top of country rankings in IPv6 adoption statistics such as Google's. If you work in an Indian ISP, telecom company or data centre, you will meet IPv6 every day.

Hexadecimal in two minutes

Writing 128 bits in binary or decimal would be very long, so IPv6 uses hexadecimal (hex), base 16. Hex uses the digits 0 to 9 and the letters a to f for 10 to 15. One hex digit is exactly 4 bits, so it is easy to convert.

The full format

An IPv6 address is eight groups of 16 bits. Each group is called a hextet and is written as four hex digits. The groups are separated by colons. For example: 2001:0db8:0000:0000:0000:ff00:0042:8329. The block 2001:db8::/32 is reserved for documentation, so you will see it in every book and in this course.

Shortening rules (RFC 5952)

1. Drop the leading zeros in each hextet: 0db8 becomes db8, 0042 becomes 42, and 0000 becomes 0. Never drop trailing zeros: db80 stays db80.
2. Replace the longest run of two or more all-zero hextets with a double colon, ::. You can do this only once in an address.
3. If two runs of zeros are equally long, shorten the leftmost one.
4. Do not use :: for a single zero hextet; write 0 instead.
5. Write the letters in lowercase.

Worked examples

Expanding a short address

To expand, count the hextets you can see and fill the :: with enough zero hextets to make eight. 2001:db8::1 shows three hextets, so :: stands for 8 − 3 = 5 zero hextets: 2001:0db8:0000:0000:0000:0000:0000:0001. This is why :: may appear only once: with two of them, 2001::1::2, nobody could tell how many zeros each one stands for.

Prefixes, /64 subnets and /48 sites

IPv6 uses prefix lengths just like CIDR: 2001:db8:acad:1::/64 means the first 64 bits are the network. Almost every LAN in IPv6 is a /64, so the last 64 bits, the interface ID, identify the device. An organisation commonly receives a /48 from its ISP (home connections often get a /56 or a /64; it varies by ISP).

With a /48 you have 16 bits of subnet ID, so 2^16 = 65,536 /64 subnets. If your site prefix is 2001:db8:acad::/48, your subnets are 2001:db8:acad:0::/64, 2001:db8:acad:1::/64, 2001:db8:acad:2::/64 and so on. Subnetting in IPv6 is usually just counting in the fourth hextet.

import ipaddress

a = ipaddress.ip_address("2001:0db8:0000:0000:0000:ff00:0042:8329")
print(a.compressed)   # 2001:db8::ff00:42:8329
print(ipaddress.ip_address("2001:db8::1").exploded)
site = ipaddress.ip_network("2001:db8:acad::/48")
print(next(site.subnets(new_prefix=64)))   # 2001:db8:acad::/64

Common mistakes

• Dropping trailing zeros: 2001:db80::1 and 2001:db8::1 are different addresses.
• Treating hex letters as mistakes: "abcd" is a normal hextet.
• Giving a LAN a prefix longer than /64. SLAAC (next topic) needs a /64 to work.