D-Link DSL-500 User Manual page 24

DSL-500 ADSL Router User's Guide
How NAT Works
In the most common NAT configuration, your network uses local IP addresses
that are not valid on the Internet. Each Internet (global) IP address is unique.
Your network administrator can assign local IP addresses on your network
(within guidelines defined later in this chapter and in "Appendix B, IP
Concepts"). This can be done manually or by using DHCP. The WAN port on the
router is assigned a globally unique public IP Address that is valid on the
Internet, since it will be sending and receiving data directly to the Internet and is
therefore part of it. Please study the example diagram below carefully.
Please note that in the above diagram, the Gateway IP address settings for the
local PC's needs to be set to 192.168.0.1, the LAN IP address of the router.
NAT manipulates the IP addresses in packet headers on a one-to-one basis. An
outgoing data packet (a packet originating from a computer on the local LAN and
destined for a computer outside the private network) will have its IP address
translated as shown below.
In the Outgoing Data Packet above, the Source IP address is the IP address that
is translated by NAT. The Destination IP Address is the IP address of a computer
outside the private network, on the Internet for example. And the Data portion of
the packet is the information payload carried by the packet, for instance a
request to view a web page.
The router logs the changes made to the IP header in its NAT table. The NAT
table enables the router to send replies back to the local computer as shown
below.
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