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)

IPv6 Datagram Delivery and Routing 1 IP NAT Overview, Motivation, Advantages and Disadvantages

To help extend the life of the IPv4 addressing scheme while the newer IPv6 protocol is developed and deployed, other technologies have been developed. One of the most important of these is IP Network Address Translation. This technology allows a small number of public IP addresses to be shared by a large number of hosts using private addresses. This essential little “trick” allows the global Internet to actually have far more hosts on it than its address space would normally support. At the same time, it provides some security benefits by making hosts more difficult to address directly by foreign machines on the public Internet.

In this section I provide a description of the concepts behind IP NAT and an explanation of operation of IP NAT types. I begin with an overview of the protocol and discussion of its advantages and disadvantages. I describe the address terminology that you need to know to understand how NAT functions and the differences between various translation techniques. I explain the way that address mappings are performed and the difference between static and dynamic address mapping.

I then explain the operation of the four main types of NAT:

1. Unidirectional NAT (also called outbound or traditional NAT)
   
   
2. Bidirectional (inbound or “two-way”) NAT
   
   
3. Port-Based or “Overloaded” NAT (also called NAPT or PAT)
   
   
4. “Overlapping” NAT (also called “Twice NAT”).

I conclude with a bit more information on compatibility issues associated with NAT.

Incidentally, most people just call this technology “Network Address Translation” without the “IP”. However, this sounds to me rather generic, and since the version being discussed here is specific to IP, I prefer to make it clear that this is an IP feature. That said, for simplicity I often just say “NAT” too, since that's shorter. I should also point out that there are quite a few people who don’t consider NAT to be a “protocol” in the strictest sense of the word.

IPv6 Datagram Delivery and Routing 1 IP NAT Overview, Motivation, Advantages and Disadvantages