Using Port Address Translation (PAT)
If you wish to enable address translation on the 700 series router, you use Port Address Translation
(PAT). PAT is a subset of NAT and is the only address translation feature on the Cisco
700 series of routers. PAT uses TCP ports to enable an entire network to use only one globally
routable IP address in the network. PAT is similar to overloading with traditional NAT.
The Cisco 700 series routers with release 4 software and higher support PAT, which enables
local hosts on an inside IP network to communicate to an outside IP network such as the Internet.
Traffic destined for an outside IP address on the other side of a border router will have its
source IP address translated before the packet is forwarded to the outside network. IP packets
returning to the inside network will have their destination IP addresses translated back to the
original source IP addresses on the inside network.
PAT conserves network addresses by enabling a single Internet-routable IP address to be
assigned to an entire LAN. All WAN traffic is usually mapped to a single IP address, which is
the ISDN-side IP address of the Cisco 700 series router. Because all the traffic on the outside network
appears to come from the Cisco 700, the inside network appears invisible to the outside
network or Internet.
You should configure a static IP address and port if remote users need to access a specific
server on the inside network. PAT will allow packets with a specific well-known port number
to get through, such as FTP or Telnet. This feature is known as a default port handler.
PAT is also sometimes referred to as NAT overload.