RD-IPtech This document discusses, for the IPv4-IPv6 coexistence period, the combined requirement that: (1) legacy IPv4 hosts can establish IPv4 transport connection s from customer sites having IPv6-only permanent addresses; (2) for good scalability, no network address translations (NATs), and a fortiori no application level gateways (ALGs), need to be supported within network infrastructures. To satisfy this requirement, it is concluded that an address-port-borrowing-protocol (APBP) is needed.