IPv4 architecture is well entrenched with Network Address Translation (NAT) boxes, which cause well-known problems for Peer-to-Peer (P2P) applications. IPv6 would enable end-to-end connectivity when deployed, but the industry has been slow in transitioning to IPv6. IPv4+4 has been suggested as an alternative NAT-extended addressing architecture, where the idea is to assign 64-bit end-to-end globally unique addresses for nodes on private address realms by concatenating the 32-bit globally routable IPv4 address of the realm (border) gateway with the 32-bit private IPv4 addresses of the nodes. While IPv4+4 addressing proposal is neat, existing IPv4+4 implementations require changes to all border gateways and end-hosts, which hinders its deployment. In this paper we show how the IPv4+4 addressing architecture can be implemented by using a modified version of the standard IPv4 Loose Source Record Route (LSRR) option. Our proposal requires no changes to existing IPv4 infrastructure (assuming all IPv4-compliant nodes implement LSRR as required by RFC 791), thus enabling seamless end-to-end communication for P2P applications. We demonstrate packet forwarding with the 64-bit IPv4+4 addresses, and illustrate how the widely-used P2P voice over IP protocol, the Session Initiation Protocol, can make use of our proposal for seamless end-to-end communication. © Springer-Verlag Berlin Heidelberg 2007.