Stateless Address Autoconfiguration (SLAAC) is an IPv6 mechanism where a host automatically configures its own global unicast address without requiring a DHCPv6 server.

IPv6 Address Format in SLAAC:

First 64 bits → Network Prefix (received from the router ' s RA message).

Last 64 bits → Interface Identifier (generated using EUI-64 or a randomly assigned value).

Why is the Prefix Always 64 Bits?

SLAAC requires a /64 prefix because the host needs the lower 64 bits to generate a unique address.

IPv6 Neighbor Discovery (ND) and Address Autoconfiguration are designed for a /64 subnet mask.

Reference from Huawei HCIE-Datacom Documentation:

Huawei IPv6 Configuration Guide – SLAAC Addressing and Prefix Allocation

HCIE-Datacom Training Material – IPv6 Stateless Address Autoconfiguration (SLAAC)