Addressing and Subnet Formats · Topic 3 of 3
IPv6 Addresses, Address Types and Prefixes
Learn how to read and shorten IPv6 addresses, the main address types such as global unicast, link local and multicast, and how prefix lengths work.
- 4 min
- An IPv6 address has 128 bits, written as eight groups of four hexadecimal digits.
- Leading zeros can be dropped, and one run of zero groups can be replaced by
::. - The double colon can be used only once in an address.
What it is
IPv6 is the newer version of the Internet Protocol, built to give every device its own address.
An IPv6 address has 128 bits, which is far more than the 32 bits of IPv4. There are enough addresses that private ranges and NAT are not needed to save space.
The address is written in hexadecimal. This page shows how to read and shorten it, the types of address, and how a prefix works.
How it works
An IPv6 address is eight groups of hexadecimal digits, and the prefix length says how many leading bits name the network.
Reading an address
The address is split into eight groups of four hexadecimal digits, separated by colons. Each group stands for 16 bits.
2001:0DB8:0000:0000:0000:0000:0000:0001Hexadecimal digits are 0 to 9 and the letters A to F. The letters can be upper or lower case.
Shortening an address
There are two rules that make addresses easier to write.
- Leading zeros in a group can be dropped. So
0DB8becomesDB8and0000becomes0. - One run of groups that are all zeros can be replaced by a double colon
::. The double colon can be used only once in an address.
Applying both rules to the address above gives a short form.
2001:DB8::1The double colon can be used once only, because otherwise you could not tell how many zero groups each one replaced.
Prefix concepts
IPv6 uses prefix length instead of a subnet mask. The notation is the same as slash notation in IPv4. In 2001:DB8:1:1::/64 the first 64 bits are the network prefix.
A normal LAN uses a /64. The first 64 bits are the prefix, and the last 64 bits are the interface ID that identifies the host. Many devices build the interface ID themselves, so a person does not need to enter each address by hand.
Types of address
IPv6 has several kinds of address, and each has a recognisable start.
| Type | Starts with | Purpose |
|---|---|---|
| Global unicast | 2000::/3, so addresses starting with 2 or 3 | Public, routable on the internet |
| Unique local | FC00::/7, in practice FD | Private use inside an organisation |
| Link local | FE80::/10 | Works only on the local link, never routed |
| Multicast | FF00::/8 | One sender to a group of receivers |
| Loopback | ::1 | A device talking to itself |
| Unspecified | :: | No address yet |
Unicast means one sender to one receiver. Multicast means one sender to a group. Anycast means one address shared by several devices, where the traffic goes to the nearest one.
IPv6 has no broadcast. Multicast takes over those jobs, for example to find other devices on the link.
An interface that runs IPv6 normally has a link local address. It is used for local tasks, such as talking to the router on the same link. The range 2001:DB8::/32 is reserved for use in documentation, so you will see it in examples.
What to remember for the exam
Seven facts cover most questions on this topic.
- An IPv6 address has 128 bits, written as eight groups of four hexadecimal digits.
- Leading zeros can be dropped, and one run of zero groups can be replaced by
::. - The double colon can be used only once in an address.
- A LAN normally uses a /64, with a 64 bit prefix and a 64 bit interface ID.
- Global unicast addresses start with 2 or 3. Link local addresses start with FE80.
- Multicast addresses start with FF. IPv6 has no broadcast.
::1is the loopback address.
To expand a short address, put back the zeros. Count the groups that are written, and the :: fills in the rest up to eight.
Common mistakes
These four errors appear again and again.
Using the double colon twice. The address becomes ambiguous and is not valid.
Dropping zeros from the end of a group. Only leading zeros can be dropped. The group 0100 becomes 100, not 1.
Looking for a subnet mask. IPv6 uses only the prefix length.
Expecting broadcast. IPv6 uses multicast for the jobs that IPv4 did with broadcast.
Do not confuse link local with unique local. Link local is not routed at all, while unique local can be routed inside an organisation.
Check yourself
Work through these three questions.
How would you shorten 2001:0DB8:0000:0000:0000:0000:0000:0001?
Drop the leading zeros and replace the run of zero groups with a double colon. The result is
2001:DB8::1.Which type of IPv6 address starts with FE80, and where is it used?
It is a link local address, used only on the local link and never routed.
Why does IPv6 not have a broadcast address?
IPv6 uses multicast for the jobs that broadcast did in IPv4.
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