What Nat Does; How Nat Works - ZyXEL Communications P-660H-61 User Manual

Chapter 9 Network Address Translation (NAT)
Table 47 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 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
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).

9.6.3 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.
Figure 68 How NAT Works
LAN
192.168.1.12
192.168.1.11
160
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
Table 48 on page 162), 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)
ADSL Router Series User's Guide
WAN