Network Address Translation devices modify the IP headers of packets between a host and server, changing some or all of the source address, destination address, source port (TCP/UDP), destination port (TCP/UDP), or ICMP query ID (for ping). The 7750 SR in both NAT modes performs Source Network Address and Port Translation (S-NAPT). S-NAPT devices are commonly deployed in residential gateways and enterprise firewalls to allow multiple hosts to share one or more public IPv4 addresses to access the Internet. The common terms of inside and outside in the context of NAT refer to devices inside the NAT (that is behind or masqueraded by the NAT) and outside the NAT, on the public Internet.
TCP/UDP connections use ports for multiplexing, with 65536 ports available for every IP address. Whenever many hosts are trying to share a single public IP address there is a chance of port collision where two different hosts may use the same source port for a connection. The resultant collision is avoided in S-NAPT devices by translating the source port and tracking this in a stateful manner. All S-NAPT devices are stateful in nature and must monitor connection establishment and traffic to maintain translation mappings. The 7750 SR NAT implementation does not use the well-known port range (1 to 1023).
In most circumstances, S-NAPT requires the inside host to establish a connection to the public Internet host or server before a mapping and translation occurs. With the initial outbound IP packet, the S-NAPT knows the inside IP, inside port, remote IP, remote port and protocol. With this information the S-NAPT device can select an IP and port combination (referred to as outside IP and outside port) from its pool of addresses and create a unique mapping for this flow of data.
Any traffic returned from the server uses the outside IP and outside port in the destination IP/port fields – matching the unique NAT mapping. The mapping then provides the inside IP and inside port for translation.
The requirement to create a mapping with inside port and IP, outside port and IP and protocol generally prevents new connections to be established from the outside to the inside as may occur when an inside host needs to be a server.