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20 C
HAPTER
2: ISDN LAN M
ODEM
F
UNCTIONALITY
D
ESCRIPTION
Call Routing Protocol
and IP Address
Translation
This section describes the call routing protocol used by the ISDN LAN Modem and
explains how IP addresses are translated.
Placing a Call to a
Previously Defined
Destination
To illustrate how the ISDN LAN Modem routes outgoing data calls, let us assume
that you have set up the following types of service providers.
■
A direct connection to an Internet Service Provider
■
A direct connection to a remote office LAN
■
A direct connection to a remote office LAN from which you can also access the
Internet
If all of those connection types are configured on the ISDN LAN Modem and are
associated with your computer, the following algorithm is performed for each of
the following scenarios.
Call Routing While No Other Calls Are Connected
If the ISDN LAN Modem has not established any calls to a remote destination and
you want to access the Internet from your computer, you simply launch your Web
browser (or whichever networking application you like). When the ISDN LAN
Modem receives the information packet requesting access to the WAN, it must
determine which connection type to use. The ISDN LAN Modem looks at the
destination Network ID (which comprises the destination IP address and subnet
mask) associated with the packet. If the Network ID of the packet matches the
Network ID of the remote LAN, with or without Internet access, then the call is
placed to the remote LAN. If it does not match the Network ID of the remote LAN,
with or without Internet access, then the call is routed to the direct ISP connection.
Once the connection is established, any authorized user on the LAN can use this
connection. The ISDN LAN Modem will translate each individual user’s IP address
into a single, shared IP address (assigned by the remote location), thereby allowing
up to 25 users to access the same remote location.
The following example shows three users sharing a connection to the Internet and
depicts the IP translation as it occurs in the ISDN LAN Modem.