Specifications

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. With no servers defined, the ADSL Router filters 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).

6.1.3 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 ADSL Router keeps track of the original addresses and port

numbers so incoming reply packets can have their original values restored. The

following figure illustrates this.