InterfaceId

InterfaceId is a term used in IPv6 networking to describe the 64-bit interface identifier that forms the suffix of an IPv6 address. An IPv6 address is typically divided into a 64-bit network prefix and a 64-bit interface identifier. The combination of the prefix and the interface identifier identifies a particular interface on a network segment, enabling routing and local reachability.

Construction methods for interface IDs have varied. Historically, many implementations derived the interface identifier from the device’s MAC address using the EUI-64 format, which inserts FF:FE in the middle of the MAC and flips the universal/local bit. Modern practice often favors privacy and security: privacy extensions generate temporary, randomly chosen interface identifiers that change over time to reduce tracking. Another approach, described in RFC 7217, produces stable, site-specific interface identifiers by hashing the network prefix, interface, and other data, yielding addresses that remain consistent within a given network while still preserving privacy.

In operation, the interface identifier enables address autoconfiguration and local communication. Through Stateless Address Autoconfiguration (SLAAC) and the Neighbor Discovery Protocol, devices acquire addresses by combining a learned or configured prefix with their interface identifier. The interface identifier thus plays a central role in address formation, neighbor reachability, and routing on IPv6-enabled networks.

Privacy and security considerations are important. Since an interface identifier can reveal hardware or user identity (when derived from a MAC), many networks prefer privacy-preserving methods or policy-based configurations to balance reachability with user anonymity.