What Nat Does; How Nat Works - ZyXEL Communications P-2900-4HB User Manual

Cable router

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Chapter 6 Network Address Translation (NAT) Screens

6.3.2 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 – see

page

65), 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).

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 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.

Figure 20 How NAT Works

Table 14 on

P-2900-4HB User's Guide

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