What Nat Does; How Nat Works - ZyXEL Communications AMG1302-T10A User Manual

Table 49 NAT Definitions (continued)
ITEM
Local
Global
NAT never changes the IP address (either local or global) of an outside host.

9.6.2 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 addre ss) 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
additional benefit of firewall protection. With no servers defined, your 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).

9.6.3 How NAT Works

Each packet has two addresses – a source address and a destination address. F or 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
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 74 How NAT Works
LAN
192.168.1.12
192.168.1.11
AMG1302-T10A User's Guide
DESCRIPTION
This refers to the packet address (source or destination) as the packet travels on the LAN.
This refers to the packet address (source or destination) as the packet travels on the WAN.
192.168.1.13
SA
192.168.1.10
Inside Local
Address (ILA)
192.168.1.10
Chapter 9 Network Address Translation (NAT)
Table 50 on page 139), NAT offers the
NAT Table
Inside Local Inside Global
IP Address
IP Address
192.168.1.10
IGA 1
192.168.1.11
IGA 2
192.168.1.12
IGA 3
192.168.1.13
IGA 4
SA
IGA1
Inside Global
Address (IGA)
WAN
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