Accessing The Wide Area Network (Wan); Network Address Translation (Nat); Ports - RTS Cronus Manual

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ACCESSING THE WIDE AREA NETWORK (WAN)

As shown in Figure 26, the LAN IP Address, 10.2.100.X (192.168.X.X is another common address), as seen in . Most devices
are shipped with these addresses as the default. It is recommended to use these addresses for LANs.
Network Address Translation
FIGURE 26.

NETWORK ADDRESS TRANSLATION (NAT)

The network address translation works using the initial IP Address, then converting it to a valid WAN IP Address. 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
locates an IP Address for the url you are requesting (http://www.telex.com). To obtain this address, the computer contacts a
DNS (Domain Name Server) 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 responding or hosting a
service is called serving.
Packet Translation
TABLE 5.
Packet before Translation
Source
IP Address
To
10.2.100.2
Internet
From 192.156.136.
Internet 22
If a second workstation on the LAN wants to communicate to the same server, and happens to use the same source port
number, then the LAN Modem translates 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, see
Figure 5.
Bosch Security Systems, Inc.
Destination
Port
IP Address
Number
192.156.136.
1031
22
80 99.5.1.30
Technical Manual
Packet after Translation
Source
Port Port
IP Address
Number Number
80 99.5.1.30 1031
192.156.136.
1031 80
22
63
Destination
Port
IP Address
Number
192.156.136.
80
22
10.2.100.2 1031
F.01U.118.890 Rev. 23