The NAT Port Mapping Protocol (NAT-PMP) is a network protocol for establishing network address translation (NAT) settings and port forwarding configurations automatically without user effort. The protocol automatically determines the external IPv4 address of a NAT gateway, and provides means for an application to communicate the parameters for communication to peers. NAT-PMP was introduced in 2005 by Apple as an alternative to the more common ISO Standard Internet Gateway Device Protocol implemented in many NAT routers. The protocol was published as an informational Request for Comments (RFC) by the Internet Engineering Task Force (IETF) in RFC 6886.
NAT-PMP runs over the User Datagram Protocol (UDP) and uses port number 5351. It has no built-in authentication mechanisms, resulting in opening itself to abuse. Protocol design treats all hosts belonging to the router's local network as trusted and allows them to freely "punch" holes through the network firewall. Though extremely convenient, such a relaxed design opens itself to easy exploiting by malicious software running on any computer that belongs to the local network, or by any rogue computers that manage to gain access to the local network. As a result, intruders can access otherwise firewalled local network services by abusing malicious "holes" punched through the firewall. Some of the NAT-PMP implementations aim to mitigate these issues by enforcing constraints to port mappings.
NAT-PMP is the precursor to the Port Control Protocol (PCP).