Netopia R2121 - Manual (Page 109)

IP Setup, SmartIP and Network Address Translation 10-5
Internal users can access the Internet as they always do; the external Internet, however, views all traffic from
the internal network as originating from 192.163.100.6. Similarly, all traffic your network receives is addressed
to 192.163.100.6.
Associating port numbers to 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 will locate 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:
■ Web ser vers typically use por t number 80
■ All FTP ser vers use por t number 21
■ 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 R2121 lets you associate these and
other por t numbers to distinct IP addresses on your internal LAN using Expor ted Ser vices. See
“IP setup” on
page 10-6 for details.
Using multiple Connection Profiles
You can enable Network Address Translation on one connection profile, disable it on another, and use the two
profiles simultaneously. The profiles might have the following attributes:
■ A profile with Network Address Translation disabled connects to your branch or main office. Your company
network administrator has assigned you a local IP address range that is consistent with the address space
assigned to your company so that you seamlessly integrate when connected. The remote IP address and
mask for this profile define only the company’s address space, so that the only IP traffic you send over this
connection is for hosts and ser vers within your company.
■ A Network Address Translation profile connects to the Internet via an ISP. Even though the ISP assigns you
a dynamic address each time you connect, there will be no address space conflict, since Network Address
Translation hides the corporate address you use locally. You enter the ISP’s remote IP address as your
default IP gateway so that any IP traffic not intended for your corporate intranet will be directed to the ISP.
Network Address Translation guidelines
Obser ve the following guidelines when using Network Address Translation.
■ The router can expor t just one local IP address per 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 TCP por t on both the ser ver and client. With two different TCP por ts