7
Accessing the Wide Area Network (WAN)
Figure 3 shows LAN IP addresses using a common IP address, 10.2.100.x (192.168.x.x is another common
address). Most devices are shipped with these addresses as its default. It is recommended to use these
addresses for LANs.
Figure 3. Network Address Translation
Network Address Translation (NAT)
Using the initial IP address, then converting it to a valid WAN IP address is how the network address transla-
tion works in theory. Once the IP address is changed, it is up to the network interface device (such as a router,
gateway, switch, etc.) to keep track of which computers are talking on which ports. For example, if two local
devices (PC1 and PC2 in Figure 3) both wanted to talk via port 1031, then the network interface device would
have to change one of the port requests to the next available port, 1032.
Ports
In general, a network port is an endpoint to a logical connection. The port number identifies what type of
port it is. For example, port 80 is used for HTTP traffic. When you type an address into the
address bar of
a web browser, your computer goes to find an IP address for the url you are requesting (http://
www.telex.com). To obtain this address, the computer contacts a DNS server (Domain Name Server).
Once the IP address is found, it tries to connect to the http port of the network device (port 80). See Table
1 for a list of the more well-known Port numbers.
Each network device can be set up to respond or not respond to the various ports. The function of re-
sponding or “hosting a service” is called “serving”.
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Table 1 Packet Translation
If a second work station on the LAN wants to communicate to the same server, and happens to use the same
source port number, then the LAN Modem will translate the source port number as well as the source IP
address. In Table 2, a second LAN computer wants to access a web page. The NAT device now uses port
1032 for this connection where it used port 1031 in Table 1.
