What is SLAAC?

SLAAC (Stateless Address Autoconfiguration) is how IPv6 hosts give themselves addresses, from a Router Advertisement's /64 prefix plus their own interface ID.

An IPv6 address is 128 bits, and on a LAN it splits into a 64-bit network prefix and a 64-bit interface identifier. SLAAC (RFC 4862) uses that split. When a host joins a network, it first gives itself a link-local address in fe80::/64, then listens for (or asks for) a Router Advertisement. If the RA contains a prefix such as 2001:db8:1000:1::/64 with the autonomous flag set, the host combines that prefix with an interface identifier it chooses itself. That identifier used to be derived from the MAC address (EUI-64); modern systems usually use a stable but opaque value instead, plus short-lived random "temporary" addresses for outgoing connections, for privacy. Before using any address, the host runs Duplicate Address Detection to check nobody else has it. The same RA gives the host its default gateway, and can include DNS servers through the RDNSS option.

"Stateless" means no server records which host has which address, unlike DHCP. That makes SLAAC simple and robust, but you can't look up a lease table to see who is using an address; you have to use the router's neighbour table or logs instead. SLAAC only works on a /64, which is why every IPv6 LAN is a /64 however few hosts it has; a /80 or /96 on a LAN breaks it. It is common to run SLAAC and DHCPv6 together, with DHCPv6 handing out other settings or delegating prefixes to downstream routers, and some platforms, notably Android, have not supported DHCPv6 address assignment, so SLAAC is the one method every host can be relied on to support. The IPv6 on VyOS lesson turns on Router Advertisements and shows hosts configuring themselves with SLAAC.