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Prestige 650ME ADSL Bridge
NAT 7-1
Chapter 7
Network Address Translation (NAT)
This chapter discusses how to configure NAT on the Prestige.
7.1 Introduction
Typically, if there are multiple users on the LAN that want concurrent access to the Internet, you must lease
a block of legal or globally unique IP addresses from an ISP. The NAT feature lets you enjoy the same
benefits as having multiple legal addresses, but only pay for one IP address, thus saving significantly on
subscription fees (check with your ISP before you enable this feature). The IP address for the NAT can be
either fixed or dynamically assigned by the ISP. In addition, you can designate servers, e.g., a web server or
a Telnet server, on your local network and make them accessible to the outside world (see below). If you do
not define a server, NAT offers the additional benefit of firewall protection. If no server is defined, all
incoming inquiries will be filtered out by your Prestige, thus preventing intruders from probing your
network. Your Prestige accomplishes address sharing by translating the internal LAN IP addresses to a
single address that is globally unique on the Internet. For more information on IP address translation, refer
to
RFC 1631 - The IP Network Address Translator (NAT).
Multiple Servers behind NAT
Even though NAT makes your whole inside network appear as a single computer to the outside world, A
service is identified by the port number; the following table shows the most common services and their
corresponding port numbers.
Table 7-1 Services and Port Numbers
SERVICES PORT NUMBER
ECHO 7
FTP (File Transfer Protocol) 21
Telnet 23
SMTP (Simple Mail Transfer Protocol) 25