3Com 3C888 User Guide - Page 24

Dial-out Call Routing, IP Address Translation, Using NAT, Network Address Translation NAT

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24 CHAPTER 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 Chrissy's PC 192.168.1.3 Larry's PC 192.168.1.4 192.168.1.2 192.168.1.4 192.168.1.5 LAN Modem translates local IP addresses to ISP-assigned IP address ISP-assigned IP address Message MODEM 1 MODEM 2 AA CD RD SD OH AA CD RD SD OH Alert Power LAN Status TX COLL 1 2 3 4 OfficeConnect Dual 56k LAN Modem 204.71.201.75 Internet 2042.07421.0.72410..7121.07.125.0715.75 Public telephone network Analog POTS line OfficeConnect Dual 56K LAN Modem 192.168.1.1 Analog POTS line Janet's PC 192.168.1.5 Figure 10 Network Address Translation in a Dial-out Connection to the Internet

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24
C
HAPTER
2: D
UAL
56K LAN M
ODEM
F
UNCTIONALITY
D
ESCRIPTION
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
Using NAT
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.
Figure 10
Network Address Translation in a Dial-out Connection to the Internet
192.168.1.5
192.168.1.2
19
2
.168.1.4
OfficeConnect
Dual 56K LAN Modem
192.168.1.1
Analog POTS line
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
204.71.201.75
204.71.201.75
204.71.201.75
204.71.201.75
ISP-assigned
IP address
LAN Modem
translates local
IP addresses to
ISP-assigned
IP address
Analog POTS line
Public telephone
network
Internet
LAN Status
MODEM 2
MODEM 1