TRENDnet TDM-C400, User Manual, Page: 118 / 205

TDM-E400 / TDM-C400 ADSL2+ Ethernet Modem Router User’s Manual
118
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 Ethernet Modem Router
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:
f
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.
f
They provide a measure of security for you LAN by
enabling you to assign private IP addresses and then have
these and the source port numbers swapped out before
your computers access the Internet.
The type of NAT function described above is called network
address port translation (
NAPT
). You can use other types, called
flavors , of NAT for other purposes; for example, providing outside
access to your LAN or translating multiple private addresses to
multiple public addresses.
For a description of
NAPT
rules, see page 125.