NAPT

NAPT, or Network Address Port Translation, is a form of network address translation used in IPv4 networks to allow multiple devices on a private network to share a single public IP address. It is a specific type of NAT that multiplexes connections by using distinct source ports for each internal host and session, effectively expanding the usable address space.

In operation, when an internal host initiates a connection to an external address, the gateway translates the private source IP and port to the public IP address and an unused external port. It records a translation entry that maps internal_ip:internal_port to public_ip:external_port for the protocol in use. The gateway rewrites the outgoing packet with the public address and port, and when a response arrives, it uses the translation table to forward it to the correct internal host and port. This allows many internal hosts to appear under a single public address while maintaining separate sessions.

NAPT supports TCP and UDP traffic and relies on a finite pool of ports (up to 65535 per protocol). Translation entries typically time out after a period of inactivity. Inbound connections from the Internet typically require explicit port forwarding rules or NAT traversal techniques; without such mechanisms, unsolicited traffic is blocked by the gateway’s filtering.

Impact and considerations include the barrier NAPT can create for end-to-end connectivity and certain protocols, as well as its lack of inherent encryption or authentication. Security is primarily provided by underlying firewall settings rather than by NAPT itself. Port exhaustion can become a limitation in networks with many simultaneous outbound connections.

In the broader context, IPv6 aims to restore end-to-end connectivity by providing globally routable addresses, reducing the need for NAT and NAPT in the long term, though NAPT remains common in many IPv4 deployments.