Netopia R9100, User Reference Manual

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IP Setup and Network Address Translation 9-5
Associating port numbers with nodes
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
■
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
“IP setup” on
page 9-6 for details.
Network Address Translation guideline
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.