IPv6 - Prefix Length used for SLAAC

In IPv6, Router Advertisements (RAs) typically contain a /64 prefix length, which is the default and most commonly used prefix. Stateless autoconfiguration (SLAAC) is designed to work exclusively with /64 prefixes. If an RA contains a prefix length other than /64, SLAAC will not function. In such cases, IPv6 addresses must be manually configured or assigned using DHCPv6.

The main reason for this is standardization and simplicity. A /64 division simplifies subnetting and makes route aggregation more efficient. Changing this would require changes to the IPv6 protocol and could lead to compatibility issues.

Links:

https://forum.networklessons.com/t/stateless-autoconfiguration-for-ipv6/897/49?u=lagapidis

https://networklessons.com/ipv6/stateless-autoconfiguration-for-ipv6