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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  IP Network Address Translation (NAT) Protocol

IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT) 1 2 3 IP NAT Compatibility Issues and Special Handling Requirements

Dealing With Overlapping Blocks By Using NAT Twice

The solution to this dilemma is to use a more sophisticated form of NAT. The other versions we have seen so far always translate either the source address or the destination address as a datagram passes from the inside network to the outside network or vice versa. To cope with overlapping addresses, we must translate both the source address and the destination address on each transition from the inside to the outside or the other direction. This technique is called Overlapping NAT in reference to the problem it solves, or “Twice NAT” due to how it solves it. (Incidentally, despite the latter name, regular NAT is not called “Once NAT”.)

Twice NAT functions by creating a set of mappings not only for the private network the NAT router serves, but also for the overlapping network (or networks) that conflict with the inside network's address space. In order for this to function, Twice NAT relies on the use of the TCP/IP Domain Name System (DNS), just like bidirectional NAT. This lets the inside network send requests to the overlapping network in a way that can be uniquely identified. Otherwise, the router can't tell what overlapping network our inside network is trying to contact.

IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT) / Port Address Translation (PAT) 1 2 3 IP NAT Compatibility Issues and Special Handling Requirements