Lesson 42 · IPv6

IPv6 addresses and SLAAC

IPv4 ran out of addresses, so IPv6 uses 128-bit ones. Learn to read and shorten them, split one into its network prefix and interface ID, tell global, link-local and multicast addresses apart, and watch a laptop build its own address from a prefix the router advertises (SLAAC).

10 min11 steps5 questionsBeginner

Fig 42 · Interactive

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5 quick questions

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01How long is an IPv6 address?

IPv6 addresses are 128 bits: eight groups of 16 bits, each written as four hex digits. IPv4 addresses are 32 bits.

02What is the correct short form of 2001:0db8:0000:0000:0000:0000:0000:0001?

Drop the leading zeros in each group (0db8 → db8, 0001 → 1), then replace the longest run of zero groups with :: once. The result is 2001:db8::1.

03Your laptop shows an address starting with fe80::. What kind of address is it?

fe80::/10 is link-local. Every IPv6 interface has one, and routers never forward packets sent to or from it.

04With SLAAC, where does the first half (the /64 prefix) of your laptop's global address come from?

The router sends the prefix (here 2001:db8:1::/64) in a Router Advertisement. The laptop adds its own 64-bit interface ID to make the full address.

05Why does a laptop use temporary (privacy) addresses?

An interface ID built from the MAC address stays the same on every network. Temporary addresses use a random interface ID and are replaced regularly (RFC 8981).

Key terms

Glossary

IPv6: Version 6 of the Internet Protocol. Its addresses are 128 bits long.
Hex digit: A digit in base 16: 0–9 then a–f. Each one stands for 4 bits.
Group: One of the eight 16-bit parts of an IPv6 address, written as up to four hex digits between colons.
Prefix (/64): The first 64 bits of an address on a LAN. It names the network, like the street in a postal address.
Interface ID: The last 64 bits of an address. It names one device's connection on that network.
Link-local address: An fe80:: address every IPv6 interface gives itself. It only works on the local link.
Multicast: Sending to a group of listeners, such as ff02::1 (all nodes) or ff02::2 (all routers). IPv6 uses it instead of broadcast.
SLAAC: Stateless address autoconfiguration: a device builds its own address from the prefix a router advertises.

Fine print

Notes and sources

IPv6 addresses are 128 bits (RFC 8200); the text form, address types and /64 interface IDs come from RFC 4291, the shortening rules from RFC 5952.
All global addresses here are inside 2001:db8::/32, reserved for documentation (RFC 3849); the MAC addresses 00:00:5e:00:53:10 and 00:00:5e:00:53:20 come from the RFC 7042 documentation block. The temporary interface ID is an example of a random value.
Building the interface ID from the MAC (modified EUI-64, RFC 4291 Appendix A) is the classic method; many systems now use a stable random-looking ID instead (RFC 7217, recommended by RFC 8064).
SLAAC is RFC 4862; Router Solicitations and Advertisements are part of Neighbor Discovery (RFC 4861). Networks can also hand out addresses with DHCPv6 (RFC 8415).

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