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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 Overview, Motivation, Advantages and Disadvantages 1 2 3 IP NAT Static and Dynamic Address Mappings

As its name clearly indicates, IP Network Address Translation is all about the translation of IP addresses. When datagrams pass between the private network of an organization and the public Internet, one or more of the addresses in these datagrams are changed by the NAT router. This translation means that every transaction in a NAT environment involves not just a source address and a destination address, but potentially multiple addresses for each of the source and destination.

In order to make more clear the explanation of how NAT operates, several special designations have been developed to refer to the different types of addresses that can be found in an IP datagram when NAT is used. Unfortunately, the terminology used for addressing in NAT can be confusing, because it's hard to visualize what the differences are between the (often similar-sounding) names. However, without knowing what these addresses mean a proper understanding of NAT operation is impossible, so we need to start by explaining them.

The first way that addresses are differentiated is based on where in the network the device is that the address refers to:

• Inside Address: Any device on the organization's private network that is using NAT is said to be on the inside network. Thus, any address that refers to a device on the local network in any form is called an inside address.
  
  
• Outside Address: The public internet—that is, everything outside the local network—is considered the outside network. Any address that refers to a public Internet device is an outside address.

An inside device always has an inside address; an outside device always has an outside address. However, there are two different ways of addressing either an inside or an outside device, depending on in which part of the network the address appears in a datagram:

• Local Address: This term describes an address that appears in a datagram on the inside network, whether it refers to an inside or outside address.
  
  
• Global Address: This term describes an address that appears in a datagram on the outside network, again whether it refers to an inside or outside address.

IP NAT Overview, Motivation, Advantages and Disadvantages 1 2 3 IP NAT Static and Dynamic Address Mappings