Dynamic NAT and Overloading

Here's how dynamic NAT works:

  • An internal network (stub domain) has been set up with IP addresses that were not specifically allocated to that company by IANA (Internet Assigned Numbers Authority), the global authority that hands out IP addresses. These addresses should be considered non-routable since they are not unique.
  • The company sets up a NAT-enabled router. The router has a range of unique IP addresses given to the company by IANA.
  • A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.
  • The router receives the packet from the computer on the stub domain.
  • The router saves the computer's non-routable IP address to an address translation table. The router replaces the sending computer's non-routable IP address with the first available IP address out of the range of unique IP addresses. The translation table now has a mapping of the computer's non-routable IP address matched with the one of the unique IP addresses.
  • When a packet comes back from the destination computer, the router checks the destination address on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address to the one saved in the address translation table and sends it to that computer. If it doesn't find a match in the table, it drops the packet.
  • The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.

Here's how overloading works:

  • An internal network (stub domain) has been set up with non-routable IP addresses that were not specifically allocated to that company by IANA.
  • The company sets up a NAT-enabled router. The router has a unique IP address given to the company by IANA.
  • A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server.
  • The router receives the packet from the computer on the stub domain.
  • The router saves the computer's non-routable IP address and port number to an address translation table. The router replaces the sending computer's non-routable IP address with the router's IP address. The router replaces the sending computer's source port with the port number that matches where the router saved the sending computer's address information in the address translation table. The translation table now has a mapping of the computer's non-routable IP address and port number along with the router's IP address.
  • When a packet comes back from the destination computer, the router checks the destination port on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address and destination port to the ones saved in the address translation table and sends it to that computer.
  • The computer receives the packet from the router. The process repeats as long as the computer is communicating with the external system.
  • Since the NAT router now has the computer's source address and source port saved to the address translation table, it will continue to use that same port number for the duration of the connection. A timer is reset each time the router accesses an entry in the table. If the entry is not accessed again before the timer expires, the entry is removed from the table.

In the next section we'll look at the organization of stub domains.