ZyXEL Communications P-660N-T1A, User Manual

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Chapter 9 Network Address Translation (NAT)
P-660N-T1A User’s Guide
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outside world. If you do not define any servers (for Many-to-One and Many-to-
Many Overload mapping – see
Table 46 on page 145 ), 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) .
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
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)