SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 60)

Configuring Network Address Translation
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Definition: An IP data packet contains bits of data bundled
together in a specific format for efficient transmission
over the Internet. Such packets are the building
blocks of all Internet communication. Each packet
contains header information that identifies
the IP address of the computer that initiates the
communication (the source IP address), the
port number that the router associates with
that computer (the source port number), the IP
address of the targeted Internet computer (the
destination IP address), and other information.
When this type of NAT rule is applied, because the source IP
address is swapped out, it appears to other Internet computers
as if the data packets are actually originating from the computer
assigned your public IP address (in this case, the ADSL Barricade).
The NAT rule could further be defined to disguise the source port in
the data packet (i.e., change it to another number), so that outside
computers will not be able to determine the actual port from which
the packet originated. Data packets that arrive in response contain
the public IP address as the destination IP address and the
disguised source port number. The ADSL Barricade changes
the IP address and source port number back to the original
values (having kept track of the changes it made earlier),
and then routes the packet to the originating computer.
NAT rules such as these provide several benefits:
•
They eliminate the need for purchasing multiple public
IP addresses for computers on your LAN. You can make
up your own private IP addresses at no cost, and then have
them translated to the public IP address when your computers
access the Internet.