 Other classifications of NAT behavior mentioned in the RFC include whether they preserve ports, when and how mappings are refreshed, whether external mappings can be used by internal hosts (i.e., its hairpinning behavior), and the level of determinism NATs exhibit when applying all these rules.Specifically, most NATs combine "symmetric NAT" for outgoing connections with "static port mapping", where incoming packets addressed to the external address and port are redirected to a specific internal address and port.The NAT traversal problem arises when two peers behind distinct NAT try to communicate.