Realm-Specific IP
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. RSIP lets a host borrow one or more IP addresses (and UDP/TCP port) from one or more RSIP gateways, by leasing (usually public) IP addresses and ports to RSIP hosts located in other (usually private) addressing realms.