3Com Dual 56K User Manual page 24

24 C 2: D 56K LAN M
HAPTER UAL
Dial-out Call Routing
IP Address Translation
Using NAT
F D
ODEM UNCTIONALITY ESCRIPTION
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.
When the Dual 56K LAN Modem is configured for any type of dial-out access, by
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
Chrissy's PC
192.168.1.3
Larry's PC
192.168.1.4
Janet's PC
192.168.1.5
Figure 10 Network Address Translation in a Dial-out Connection to the Internet
LAN Modem
translates local
IP addresses to
ISP-assigned
ISP-assigned
IP address
IP address
204.71.201.75
MODEM 1 MODEM 2 LAN Status
OfficeConnect Dual 56k LAN Modem
OfficeConnect
Dual 56K LAN Modem
192.168.1.1
Analog POTS line
Internet
Analog POTS line
Public telephone
network