464XLAT
IPv4-on-IPv6-only-network transition mechanism. RFC 6877 (2013). Combines client-side stateless translation (CLAT) + network-side stateful translation (PLAT/NAT64) for apps that are hardcoded-IPv4-only.
Client (phone): runs a mini-NAT internally (CLAT) that maps app-IPv4 traffic to IPv6 source. Network: NAT64 maps IPv6 back to real IPv4 at the edge. Result: legacy IPv4 apps work transparently on an IPv6-only network. T-Mobile USA cellular uses 464XLAT — phones run CLAT internally. Modern Android (10+) has built-in CLAT. iOS native-IPv6-only apps don't need CLAT, but legacy apps benefit. Performance overhead: minimal (in-kernel translation), but adds complexity. Future irrelevance: as IPv6 adoption climbs, fewer apps need IPv4, 464XLAT mostly handles legacy.