ZyXEL Communications Prestige 662HW Series, Руководство пользователя

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Prestige 662HW Series User’s Guide
8-2 Network Address Translation (NAT) Screens
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.