ATL Telecom AM30 - Manual (Page 62)

62 ATL Telecom User Guide
AM30
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 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:
fThey 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.
fThey 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 68.