A port number identifies a TCP or UDP application endpoint and helps distinguish concurrent communication sessions on one host. Dynamic PAT uses this identifier to let many inside hosts share one public IPv4 address. section 9.3.3
Dynamic PAT uses port numbers as follows:
- The router translates inside local IP addresses and, when necessary, TCP or UDP port numbers. section 9.3.3
- A unique translated port number distinguishes each simultaneous session sharing an inside global address. section 9.3.3
- There are 65,536 possible port numbers, although practical capacity depends partly on router memory and other factors. section 9.3.3
- The router preserves the original port when possible, changes it when already allocated, and keeps the chosen translation for the session. section 9.3.3
- ICMP has no TCP or UDP ports; translated ICMP messages use the 16-bit Identifier field for a similar tracking role. section 9.3.3
PAT translation table
A router can map 10.0.0.11:44578, 10.0.0.12:33032, and 10.0.0.13:39102 to the same public address, 203.0.113.2, while retaining distinct port numbers for each TCP session. section 9.3.3
R1(config)# interface g0/0 R1(config-if)# ip nat inside R1(config-if)# interface g0/1 R1(config-if)# ip nat outside R1(config-if)# exit R1(config)# access-list 1 permit 10.0.0.0 0.0.0.255 R1(config)# ip nat pool POOL1 192.0.2.1 192.0.2.5 prefix-length 29 R1(config)# ip nat inside source list 1 pool POOL1 overload #1 #1 Adds the overload keyword to the NAT statement
Configure PAT overload with inside and outside interfaces, an access list for inside addresses, and either a NAT pool or an interface address. section 9.3.3
