RTS XCP-48-RJ45 Bedienungsanleitung PDF herunterladen (Seite 65)

ACCESSING THE WIDE AREA NETWORK (WAN)

Figure 13 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.

NETWORK ADDRESS TRANSLATION (NAT)

Using the initial IP Address, then converting it to a valid WAN IP Address is how the network address translation 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 responding or “hosting a

service” is called “serving”.

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 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.

Figure 3. Network Address Translation

TABLE 1. Packet Translation

Packet before Translation Packet after Translation

Source Destination Source Destination

IP Address

Port

Number

IP Address

Port

Number

IP Address

Port

Number

IP Address

Port

Number

Internet

10.2.100.2 1031

192.156.136.

80 99.5.1.30 1031

192.156.136.

From

Internet

192.156.136.

80 99.5.1.30 1031

192.156.136.

80 10.2.100.2 1031

1 2 ... 60 61 62 63 64 65 66 67 68 69 70 ... 106 107

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