ParkerVision WR1500 - Manual (Page 69)

WR1500
4-Port Wireless DSL/Cable Router
®
71
9.1 NAT Overview - Continued
9.1.2 What NAT Does
In the simplest form, NAT changes the source IP address in a packet received from a subscriber
(the inside local address) to another (the inside global address) before forwarding the packet
to the WAN side. When the response comes back, NAT translates the destination address (the
inside global address) back to the inside local address before forwarding it to the original inside
host. Note that the IP address (either local or global) of an outside host is never changed.
The global IP addresses for the inside hosts can be either static or dynamically assigned by
the ISP. In addition, you can designate servers (for example a web server and a telnet server)
on your local network and make them accessible to the outside world. Although you can make
designated servers on the LAN accessible to the outside world, it is strongly recommended that
you attach those servers to the DMZ port instead. If you do not defi ne any servers (for Many-to-
One and Many-to-Many Overload mapping), NAT offers the additional benefi t of fi rewall
protection. With no servers defi ned, your WR1500 Wireless Router fi lters out all incoming
inquiries, thus preventing intruders from probing your network. For more information on IP
address translation, refer to RFC 1631, The IP Network Address Translator (NAT).
9.1.3 How NAT Works
Each packet has two addresses - a source address and a destination address. For outgoing
packets, the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside
Global Address) is the source address on the WAN. For incoming packets, the ILA is the
destination address on the LAN, and the IGA is the destination address on the WAN. NAT
maps private (local) IP addresses to globally unique ones required for communication with
hosts on other networks. It replaces the original IP source address (and TCP or UDP source
port numbers for Many-to-One and Many-to-Many Overload NAT mapping) in each packet and
then forwards it to the Internet. The WR1500 Wireless Router keeps track of the original
addresses and port numbers so incoming reply packets can have their original values restored.
The following fi gure illustrates this.