ZyXEL Communications Prestige 861H-G1, Руководство пользователя

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Prestige 861H User’s Guide
8-2 Network Address Translation (NAT) Screens
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 8-2 ), NAT offers the
additional benefit of firewall protection. With no servers defined, your Prestige 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) .
8.1.3 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 Prestige 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 8-1 How NAT Works
8.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical
LANs using IP Alias) behind the Prestige can communicate with three distinct WAN networks.
More examples follow at the end of this chapter.