ZyXEL Communications AMG1302-T11C - How NAT Works; NAT Application

Chapter 11 Network Address Translation (NAT)
AMG1302-T11C User’s Guide
155
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 56 on page 157), NAT offers the
additional benefit of firewall protection. With no servers defined, your AMG1302-T11C 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).
11.6.3 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
AMG1302-T11C 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
11.6.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical LANs
using IP alias) behind the AMG1302-T11C can communicate with three distinct WAN networks.
192.168.1.13
192.168.1.10
192.168.1.11
192.168.1.12
SA
192.168.1.10
SA
IGA1
Inside Local
IP Address
192.168.1.10
192.168.1.11
192.168.1.12
192.168.1.13
Inside Global
IP Address
IGA 1
IGA 2
IGA 3
IGA 4
NAT Table
WAN
LAN
Inside Local
Address (ILA)
Inside Global
Address (IGA)