M.I.T. This document discusses the operating principles of Network Address Translator (NAT) devices and the behavioral properties required to make NAT more predictable and compatible with diverse application protocols. First, this document presents an architectural model for NAT devices and defines important terms used in conjunction with NAT operation. The architectural model sets the stage for a set of concrete recommendations for NAT implementers. The recommendations made by this document are independent of transport protocol. A set of companion documents provide behavioral recommendations specific to particular transport protocols.