Realm-Specific IP was an experimental IETF framework and protocol intended as an alternative to network address translation (NAT) in which the end-to-end integrity of packets is maintained.
The RSIP host uses this address to send packets to destinations in the other realm.
The tunnelled packets between RSIP host and gateway contain both addresses, and the RSIP gateway strips off the host address header and sends the packet to the destination.
RSIP should be useful for NAT traversal as an IETF standard alternative to Universal Plug and Play (UPnP).
As of November 2004[update], the protocol was in the experimental stage and not yet in widespread use.