Prestige 642 PPPoE Modem

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 and 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 3-7 Common Services and Corresponding Port Numbers

SERVICES

PORT NUMBER

FTP (File Transfer Protocol)

21

 

 

Telnet

23

 

 

SMTP (Simple Mail Transfer Protocol)

25

 

 

DNS (Domain Name System)

53

 

 

HTTP (Hyper Text Transfer Protocol or WWW Web)

80

 

 

PPTP (Point-to-Point Tunneling Protocol)

1723

 

 

So, if you have a web server at IP address 192.168.1.2 and a FTP server at IP address 192.168.1.3, then you need to specify port 80 (web) for the server at IP address 192.168.1.2 and port 21 (FTP) for the FTP server at IP address 192.168.1.3.

A server can support more than one service, e.g., a server can provide both FTP and DNS service, while another provides only web service. Furthermore, since you need to specify the IP address of a server in the Prestige, a server must have a fixed IP address and not be a DHCP client whose IP address potentially changes each time it is turned on.

In addition to servers for specific services, NAT supports a default server. A service request that does not have a server explicitly designated for it is forwarded to the default server. If the default server is not defined, the service request is simply discarded. To make a server visible to the outside world, specify the port number of the service in the Port Number field and the inside IP address of the server in the Server IP Address field.

The Prestige has some of the more popular applications already pre-configured. Select an application from the drop-down list box and the corresponding port number should display in the Port Number field. To

Web Configurator

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