(Realm Specific IP) A network address translation technology that performs a similar function to NAT while maintaining end-to-end transparency. Whereas NAT inserts new IP addresses into the packet headers at the router, RSIP does it at the client side. When a user's machine (RSIP client) wants to contact a host on the Internet, it queries the RSIP server for a port number and public IP address. The client then tunnels the packets to the RSIP server, which strips off the tunnel headers and sends the packets to the Internet. On incoming packets, the RSIP server looks up the client IP based on port number, adds the tunnel header and sends them to the RSIP client. See RSIP gateway and NAT.