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Configuring Network Address Translation
PR2000 Installation Guide
47
7 Configuring Network Address Translation
In this Chapter
This chapter explains how to setup Network Address Translation (NAT). NAT is
used to map internal IP addresses in the local network to different IP addresses
in the external network. You can skip this chapter if NAT is not being used in
your configuration.
7.1 What is NAT?
Network Address Translation (NAT) is a mechanism defined by the RFC1631
that was created in order to address the problem of IP address depletion in the
Internet and allow hosts in a LAN to share IP addresses.
When NAT is implemented in a router connecting a network to the Internet, it
allows internal hosts to use non-registered IP addresses by mapping them to
"legal" or registered IP addresses when the connections go outside the local
network. This way, assuming that not all the hosts in the network are
simultaneously connected to the external world, NAT allows a potentially large
network to use only a small pool of valid IP addresses.
Another instance of NAT is port mapping. In this case, the local TCP or UDP
port number is also mapped and the pair address/local port defines the
connection. This allows one global IP address to be used more than once to
map different connections. It is particularly useful when the router is being used
to connect a small LAN (in a small office/home office configuration) to a dialup
line (which normally gets only one IP address assigned to it).
Because NAT allows the internal IP addresses to be hidden from the external
world, it is sometimes also seen as a security mechanism.
The private network using non-registered (or local) IP addresses is called the
"Local" network. The "Global" network is the external network (typically the
Internet), using registered (or global) IP addresses.
NAT keeps a table that associates the local addresses with global addresses.
Because the local addresses are not visible to the external world, a mapped