Alcatel-Lucent Cisco Systems This document describes a "Layer2-Aware" IPv4-to-IPv4 (NAT44) Service Provider NAT function that identifies subscriber traffic based on IP- independent methods such as a link-layer address, VLAN, PPP session, tunnel, etc. in order to allow one to either avoid "double-NAT" (NAT444) of subscriber IP traffic altogether, the need for additional "Shared Service-Provider" IPv4 address space, or partitioning of RFC 1918 space between subscribers. While the mechanisms described in this document may be applicable to a variety of network architectures, the primary focus is on residential "fixed-line" Internet access.