3Com Dual 56K manual 192

24CHAPTER 2: DUAL 56K LAN MODEM FUNCTIONALITY DESCRIPTION

Dial-out Call Routing When the LAN Modem makes an outgoing call, it follows a specific order for routing data packets, based on the destination Network ID (that is, the IP address and subnet mask) associated with the packet. If you plan to configure more than one Service Provider in the LAN Modem, and in particular if you are configuring both an Internet Service Provider and a Private Network Service Provider that includes Internet access, you may want to understand how routing order is determined in the LAN Modem.

When the LAN Modem receives a packet requesting WAN access, it looks first to see whether the Network ID of the packet matches the Network ID of a configured Private Network. If the Network ID of the packet matches the Network ID of the Private Network, then the call is placed to the Private Network; if it does not match the Network ID of the Private Network, then the call is routed to the first configured ISP connection.

IP Address Translation When the Dual 56K LAN Modem is configured for any type of dial-out access, by Using NAT default the IP address translation system, Network Address Translation (NAT),

is used to enable IP address sharing among the attached workstations.

NAT works by taking the local, private (i.e., not “publicly-routable”) IP addresses of individual workstations attached to the LAN and translating them into a single, publicly-routable IP address assigned by the remote location and used specifically for communication across the WAN. Unlike a publicly-routable IP address, a local IP address cannot be used for communication across the WAN and is functional only within the boundaries of the LAN.

By translating the local IP addresses of each workstation in this way, the LAN Modem can send out all packets destined for the same remote location over an already-established connection. The LAN Modem in effect “masquerades” as a single user in its communication with the remote, call-receiving device.

The following example shows IP address translation as it occurs in the LAN Modem when three users share a single connection to the Internet.

Jack’s PC 192.168.1.2

	192
	.
	168
	.
Chrissy’s PC	1
	.
	2
192.168.1.3
	.4
	.1
	.168
Larry’s PC	2
	19
192.168.1.4	.5
	.1
	.168
	192

Janet’s PC 192.168.1.5

LAN Modem						Internet
translates local			.75
IP addresses to					5
		.201
ISP-assigned	ISP-assigned			.7		5
		.71	.201 .
IP address		204 .71			7
	IP address			.201
		204	.71
			4
		20
	204.71.201.75	Public telephone		Analog POTS line
MODEM 1MODEM 2LAN Status
		network
OfficeConnect
Dual 56K LAN Modem
192.168.1.1	Analog POTS line

Figure 10 Network Address Translation in a Dial-out Connection to the Internet