ZyXEL Communications eircom D1000, User Manual

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125
eircom D10
00 modem User’s Guide
9
Network Address Translation (NAT)
9.1 Overview
This chapter discusses how to configure NAT on the Device. 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.
9.1.1 What You Can Do in the NAT Screens
• Use the General screen ( Section 9.2 on page 126 ) to activate/deactivate NAT for the default
WAN connection (PVC0).
• Use the Port Forwarding screen ( Section 9.3 on page 127 ) to configure forward incoming
service requests to the server(s) on your local network.
• Use the DMZ screen to configure a default server ( Section 9.4 on page 129 ).
• Use the ALG screen to enable and disable the SIP (VoIP) ALG in the Device ( Section 9.5 on page
130 ).
9.1.2 What You Need To Know About NAT
Inside/Outside
Inside/outside denotes where a host is located relative to the 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
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.