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11-2 Administration Guide
The following is a general description of these features:
Port Address Translation
The simplest form of classic Network Address Translation is PAT (Port 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 surf 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.
Server lists
Server lists, previously known as exported services, make it possible to provide access from the public network
to hosts on the LAN. Server lists allow you to define particular services, such as Web, ftp, or e-mail, which are
available via a public IP address. You define the type of service 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 service if you want to use an IP other than the WAN IP address of the Netopia Router.
Static mapping
If you want to host your own Website or provide other Internet services to the public, you need more than
classic NAT. The reason is noted under Port 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 server that you host, you
need to make a public representation of the real IP addresses of those servers.
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
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 necessary.