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The following is a general description of these features:

The simplest form of classic Network Address Translation is PAT (Por t Address Translation). PAT allows a group 
of computers on a LAN, such as might be found in a home or small office, to share a single Internet connection 
using one IP address. The computers on the LAN can sur f the Web, read e-mail, download files, etc., but their 
individual IP addresses are never exposed to the public network. Instead, a single IP address acts as the 
source IP address of traffic originating from the LAN. The Netopia Router allows you to define multiple PAT 
mappings, which can be individually mapped to different public IP addresses. This offers more control over the 
access permitted to users on the LAN.

A limitation of PAT is that communication must be initiated from the internal network. A user on the external 
side cannot access a machine behind a PAT connection. A PAT enhancement introduced in firmware version 4.4 
is the ability to define multiple PAT mappings. Each of these can optionally map to a section or range of IP 
addresses of the internal network. PAT mapping allows only internal users to initiate traffic flow between the 
internal and external networks.

Ser ver lists, previously known as expor ted ser vices, make it possible to provide access from the public network 
to hosts on the LAN. Ser ver lists allow you to define par ticular ser vices, such as Web, ftp, or e-mail, which are 
available via a public IP address. You define the type of ser vice you would like to make available and the 
internal IP address to which you would like to provide access. You may also define a specific public IP address 
to use for this ser vice if you want to use an IP other than the WAN IP address of the Netopia Router.

If you want to host your own Website or provide other Internet ser vices to the public, you need more than 
classic NAT. The reason is noted under Por t Address Translation above – external users cannot initiate traffic to 
computers on your LAN because external users can never see the real addresses of the computers on your 
LAN. If you want users outside your LAN to have access, for example, to a Web or FTP ser ver that you host, you 
need to make a public representation of the real IP addresses of those ser vers.

Static mappings are a way to make one or more private IP addresses fully accessible from the public network 
via corresponding public IP addresses. Some applications may negotiate multiple TCP connections in the 
process of communication, which often does not work with traditional PAT. Static mapping offers the ability to 
use these applications through NAT. Each private IP address is mapped, on a one-to-one basis, to a public IP 
address that can be accessed from the Internet or public network. As with PAT mappings, you may have multiple 
static mappings to map a range of private IP addresses to a range of public IP addresses if desired.

Dynamic mapping, often referred to as many-to-few, offers an extension to the advantages provided by static 
mapping. Instead of requiring a one-to-one association of public addresses and private addresses, as is 
required in static mapping, dynamic mapping uses a group of public IP addresses to dynamically allocate static 
mappings to private hosts that are communicating with the public network. If a host on the private network 
initiates a connection to the Internet, for example, the Netopia Router automatically sets up a one-to-one 
mapping of that host’s private IP address to one of the public IP addresses allocated to be used for Dynamic 
NAT. As long as this host is communicating with the Internet, it will be able to use that address. When traffic 
from that host ceases, and no traffic is passed from that host for five minutes, the public address is made 
available again for other private hosts to use as necessar y.