Farallon Communications R9100 Manuel D’Utilisation

When an IP client such as a Netscape Navigator or Microsoft Internet Explorer, wants to establish a session 
with an IP ser ver such as a Web ser ver, the client machine must know the IP address to use and the TCP 
ser vice por t where the traffic is to be directed.

For example, a Web browser locates a Web ser ver by using a combination of the IP address and TCP por t that 
the client machine has set up. Just as an IP address specifies a par ticular computer on a network, por ts are 
addresses that specify a par ticular ser vice in a computer. There are many universally agreed-upon por ts 
assigned to various ser vices. For example:

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Web ser vers typically use por t number 80 

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All FTP ser vers use por t number 21

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Telnet uses por t number 23

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SNMP uses por t number 161

To help direct incoming IP traffic to the appropriate ser ver, the Netopia R9100 lets you associate these and 
other por t numbers with distinct IP addresses on your internal LAN using expor ted ser vices. See 

 for details.

Obser ve the following guideline when using Network Address Translation.

The router can expor t only one local IP address per UDP/TCP por t, so you can have just one machine available 
for a given ser vice, such as one FTP ser ver. However, some ser vices, such as Web ser vers (www-http ser vers), 
allow you to change the UDP/TCP por t on both the ser ver and client. With two different UDP/TCP por ts 
expor ted, you can have Web ser vers on two different IP hosts.