ZyXEL Communications P-660HN-F1, User Manual

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ADSL Series User’s Guide
175
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H A P T E R
11
Network Address Translation (NAT)
11.1 Overview
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet, for example, the source address of an outgoing packet, used within one network to a
different IP address known within another network.
11.1.1 What You Can Do in this Chapter
• Use the Port Forwarding screen to configure forward incoming service requests to the server(s)
on your local network ( Section 11.2 on page 176 ).
• Use the Sessions screen to limit the number of concurrent NAT sessions each client can use
( Section 11.3 on page 178 ).
• Use the ALG screen ( Section 11.4 on page 179 ) to enable and disable the SIP (VoIP) ALG in the
ZyXEL Device.
11.1.2 What You Need To Know
The following terms and concepts may help as you read this chapter.
Inside/Outside and Global/Local
Inside/outside denotes where a host is located relative to the ZyXEL Device, for example, the
computers of your subscribers are the inside hosts, while the web servers on the Internet are the
outside hosts.
Global/local denotes the IP address of a host in a packet as the packet traverses a router, for
example, the local address refers to the IP address of a host when the packet is in the local
network, while the global address refers to the IP address of the host when the same packet is
traveling in the WAN side.
NAT
In the simplest form, NAT changes the source IP address in a packet received from a subscriber
(the inside local address) to another (the inside global address) before forwarding the packet to the
WAN side. When the response comes back, NAT translates the destination address (the inside
global address) back to the inside local address before forwarding it to the original inside host.