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TW-H6W1IR ISDN Remote Router
Configuration and Management 81
NAT is the initial protocol set forth by RFC 1631 and provides a
means in which private networks can communicate with the Internet by
using a small number of IP addresses. In our discussion, we will use the
example IP addresses listed in the table below and the network diagram
shown on page 78.
Global IP Addresses
(for use with NAT) Local IP Addresses
(assigned to computers
on the local network)
200.100.50.1 192.168.100.2
200.100.50.2 192.168.100.3
200.100.50.3 192.168.100.4
200.100.50.4 192.168.100.5
200.100.50.5 192.168.100.6
192.168.100.7
192.168.100.8
192.168.100.9
192.168.100.10
Please note that in the above table there are 9 users on the local
network using 5 global IP addresses to access the Internet.
When a packet on the local network arrives at the router and needs to
be sent to the Internet, NAT will change the source IP address (for
example 192.168.100.2) to a global address (200.100.50.1, for
example). If this packet generates a reply (as for example, a request to
view a web page will), NAT will change the destination IP address on
the reply packet back to the local IP address for delivery to the
machine on the local (stub) network.