Proxicast LAN-Cell 2 User's Guide download pdf (Page 303)

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Chapter 13 Network Address Translation (NAT) Screens

LAN-Cell 2 User’s Guide

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that you attach those servers to the DMZ port instead. If you do not define any servers (for

Many-to-One and Many-to-Many Overload mapping), NAT offers the additional benefit of

firewall protection. With no servers defined, your LAN-Cell 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).

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 LAN-Cell keeps track of the original addresses and

port numbers so incoming reply packets can have their original values restored. The following

figure illustrates this.

" NAT never changes the IP address (either local or global) of an outside host.

Figure 181 How NAT Works

NAT Application

The following figure illustrates a possible NAT application, where three inside LANs (logical

LANs using IP Alias) behind the LAN-Cell can communicate with three distinct WAN

networks. More examples follow at the end of this chapter.

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