Network Address Translation (NAT)
NN40011-046 Issue 1.2 BCM50 Rls 6.0 5
Network Address Translation (NAT)
Overview
Many people view the Internet as a "one-way street"; they forget that while
their computer is connected to the Internet, the Internet is also connected to
their computer. That means that anybody with Net access can potentially
access resources on their computers (such as files, email, company network
etc). Most personal computer operating systems are not designed with
security in mind, leaving them wide open to attacks from the Internet.
Network address translation (NAT) is a scheme that allows two connected
networks (for example your Private LAN connected to the Internet) to use
different and incompatible IP addressing schemes. Address translation allows
hosts on a private internal network to transparently communicate with
destinations on an external network or vice versa. In this way, NAT is being
used as a security mechanism to hide the internal IP addresses.
This also means that NAT allows the connecting of multiple computers to the
Internet (or any other IP network) using one IP address, providing small
businesses the means to connect their network to the Internet cheaply and
efficiently.
Note: This guide relates to the BCM50a/ba and BCM50e/be models only.
Note: Although the BCM50a/ba models will not be supplied with BCM 6.0, it is
possible to upgrade the variants of these models to BCM 6.0, if they were
originally supplied with BCM50 R2 or BCM50 R3 software.
Note: The BCM50 Integrated Router is almost identical to the Business
Secure Router (BSR) models. BCM50a/ba routers are based on the BSR252
and BCM50e/be routers are based on the BSR222.
BCM50 and NAT
The BCM50 Integrated Router enables a LAN network consisting of multiple
computers to access the Internet, even though there may only be a single or a
few WAN IP Addresses available. The process is roughly described as
follows:
A PC on the network makes a request for information from the Internet
The BCM50 Integrated Router keeps a track of IP and port information
for the outgoing packets, and replaces the source information with its
own WAN information
Returning packets are checked and the original IP and port information
is applied, to ensure that the packets return to the originator
BCM50 Integrated Router NAT is able to perform with either a single ISP-
allocated WAN IP address, or multiple ISP-allocated WAN IP addresses. If a
