ZyXEL Communications P-324 User's Guide download pdf (Page 154)

P-2602HWNLI User’s Guide

154 Chapter 10 Network Address Translation (NAT) Screens

10.1.2 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. If you do not define any

servers (for Many-to-One and Many-to-Many Overload mapping – see Table 50 on page 156),

NAT offers the additional benefit of firewall protection. With no servers defined, your ZyXEL

Device 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).

10.1.3 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 ZyXEL Device keeps track of the original addresses

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

following figure illustrates this.

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ZyXEL Communications P-324 User's Guide Page 154