How Nat Works - ZyXEL Communications P-660N-T1A User Manual

outside world. If you do not define any servers (for Many-to-One and Many-to-
Many Overload mapping – see
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 71 How NAT Works
LAN
192.168.1.12
192.168.1.11
P-660N-T1A User's Guide
Table 46 on page
192.168.1.13
SA
192.168.1.10
Inside Local
Address (ILA)
192.168.1.10
Chapter 9 Network Address Translation (NAT)
145), NAT offers the additional
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
143