Network Address Translation (NAT) is a solution for a number of problems: depletion of IP addresses, migration from an Internet service provider (ISP), merging networks, and destination load balancing.
NAT advantages are address conservation, flexibility, overlapping networks, ease of IP renumbering, and security. Disadvantages are latency, functionality, traceability, and resource use.
You use a static mapping to make a resource on the private side of your network available to the outside.
Using dynamic NAT, you can convert inside addresses to a range of outside addresses. You use a standard access list to specify the addresses that you want converted and a nat pool command to specify the range of addresses to be translated to.
With NAT overloading, you specify that you want all private inside addresses translated to a single outside global address. You add the keyword overload to the end of the ip nat inside source command.
NAT overlapping is where the same addresses are used on two different networks, and the networks are trying to reach each other.
show ip nat translation lets you see that NAT is functioning, and clear ip nat translation * drops all translations.
Interfaces are identified as either inside or outside. NAT can be done in either direction, but traffic must flow from an inside interface to an outside one, or vice versa.
An access list identifies addresses that may be translated. The access list is linked to a NAT pool through a pool name.
Port address translation (PAT), or NAT overload, can cause connectivity problems in applications that require specific port numbers on each side of the session. NAT problems can occur when an application inserts the source IP address into the packet and the address gets changed.
