Complex sentence: 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.
Split into simpler sentences: RSIP lets an RSIP host borrow one or more IP address (and UDP/TCP port) from one or more RSIP gateway. RSIP functions by leasing (usually public) IP addresses and ports to RSIP hosts located in other (usually private) addressing realms.