ZyXEL Prestige 642 User Manual download pdf (Page 37)

Prestige 642 PPPoE Modem

Web Configurator 3-15

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

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ZyXEL Prestige 642 User Manual Page 37