NAT Terminology

One of the biggest challenges to learning and understanding NAT is the terms that are used to describe the different concepts and configurations. The terms that cause the most confusion are associated with the addressing: inside, outside, local, and global. More than one of these terms can apply to an address, depending on your perspective:

• Inside or outside Specifies the physical location of a host based on the device performing NAT.

• Local or global Specifies the client's point of view based on the NAT device.

Inside and local refer to the same side of the NAT device, typically called the private or internal network. Outside and global refer to the other side of the NAT device, known as the public or external network. The four possible combinations are described in Table 5.1.

Table 5.1. NAT Terminology

Term	Explanation
Inside local address	The IP address assigned to a host on the private or internal network. Usually based on RFC 1918.
Inside global address	A legitimate address on the public or external network. Usually provided by your ISP. This translated address is viewable to the outside world that maps back to your inside local address.
Outside global address	Someone else's inside global address. An address of an external host on the public network. A routable address provided by the ISP.
Outside local address	An IP address of an outside host as it appears to the private or internal network. Not necessarily a legitimate address, it is allocated from the inside address space. Usually based on RFC 1918.

RFC 1918 sets aside network ranges to be used for private networks:

• 10.0.0.0 to 10.255.255.255

• 172.16.0.0 to 172.31.255.255

• 192.168.0.0 to 192.168.255.255