SLAAC, EUI-64 and IPv6 Subnetting

What it is

SLAAC lets a host build its own IPv6 address from a router advertisement, and subnetting splits a prefix into networks.

SLAAC stands for Stateless Address Autoconfiguration. The host learns a /64 prefix from a router and adds an interface ID that it creates itself. No server keeps a record of the address, which is why it is called stateless.

EUI-64 is one way to build the interface ID, using the MAC address of the interface. Subnetting works on the other half of the address, the part that identifies the network.

How it works

A 128 bit address is split into a network prefix and a 64 bit interface ID.

SLAAC step by step

The steps are the same on every host:

1. The host creates a link local address, which starts with fe80::.
2. It checks that the address is free with Duplicate Address Detection.
3. It sends a Router Solicitation, or waits for a periodic Router Advertisement.
4. The router advertises a prefix with the autonomous flag set.
5. The host joins the prefix and its own interface ID to make a global address, and checks it for duplicates too.

SLAAC works only with a /64 prefix, because the interface ID is 64 bits long. The advertisement also tells the host the router address to use as a default gateway.

Building the interface ID with EUI-64

EUI-64 turns the 48 bit MAC address into a 64 bit interface ID. There are two changes to the MAC address:

• Insert ff:fe in the middle of the MAC address.
• Flip the seventh bit of the first byte, called the universal or local bit.

Take the MAC address 00:0c:42:aa:bb:cc. Splitting it in the middle and inserting fffe gives 000c:42ff:feaa:bbcc. Flipping the seventh bit changes the first byte from 00 to 02.

MAC           00:0c:42:aa:bb:cc
insert fffe   000c:42ff:feaa:bbcc
flip bit 7    020c:42ff:feaa:bbcc

With the prefix 2001:db8:1:1::/64, the address becomes 2001:db8:1:1:20c:42ff:feaa:bbcc. Leading zeros drop out when it is written.

A RouterOS address can use this method too. The eui-64 option builds the interface ID from the MAC address of the interface.

/ipv6 address add address=2001:db8:1:1::/64 interface=ether2 eui-64=yes

Subnetting an IPv6 prefix

IPv6 subnetting is easier than IPv4, because you rarely worry about running out of host addresses. You decide how many bits to use for subnets, then count in hex.

Suppose a site receives 2001:db8:abcd::/48. A /64 per segment leaves 16 bits for the subnet ID, which is 65536 subnets.

The subnet ID is the fourth group. Counting up in hex gives the next subnet. Every subnet keeps 64 bits for hosts.

Subnet lengths that fall on a multiple of four bits are easy to read, because each hex digit is four bits. A /52 changes the first digit of the fourth group. A /56 changes the first two digits.

On a point to point link, a /64 is still the safest default. Smaller prefixes on links are possible but can break features that expect a /64.