Network Address and Port Translation IPv6 to IPv4

Description

NAPT64 operates as a stateful network address and port translation function, typically deployed at the boundary between an IPv6-only access network and the IPv4 internet. Its architecture includes a translator that intercepts packets from IPv6 hosts destined for IPv4 servers. The translator maintains a binding table that maps the IPv6 source address and port to a synthesized IPv4 address and port from a pool of available IPv4 addresses. When an IPv6 host initiates a session to an IPv4 destination, the NAPT64 device creates a mapping entry. It rewrites the IPv6 packet header, converting the source IPv6 address/port to an IPv4 address/port from its pool and the destination IPv4 address to a synthesized IPv6 address (often using a predefined IPv6 prefix like 64:ff9b::/96 for well-known IPv4 addresses). The return traffic from the IPv4 server is translated back using the binding table, ensuring the IPv6 host receives the response. Key components include the translator function, the IPv4 address pool, and the binding state table for session management. Its role is critical in dual-stack lite (DS-Lite) and other IPv6 transition architectures, acting as a carrier-grade NAT (CGN) for IPv6 to IPv4 traffic, thus conserving scarce IPv4 addresses and facilitating a smoother transition to IPv6 networks.

Purpose & Motivation

NAPT64 was created to address the exhaustion of IPv4 addresses and the gradual migration to IPv6 networks. As operators began deploying IPv6-only access networks, a significant challenge emerged: how to allow these IPv6-only clients to access the vast majority of internet services still hosted on IPv4. Traditional dual-stack (supporting both IPv4 and IPv6 on every device) was not always feasible due to IPv4 address scarcity. NAPT64 solves this by enabling IPv6-only devices to communicate with IPv4 servers without requiring a full IPv4 stack on the client, thus simplifying network architecture and reducing operational costs. It is a key component in IPv6 transition strategies, allowing for the phased decommissioning of IPv4 infrastructure while maintaining service continuity.