ZyXEL Communications SBG5500-A, User Manual

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SBG5500 Series User’s Guide
122
C
HAPTER
8
Network Address Translation
(NAT)
8.1 Overview
This chapter discusses how to configure NAT on the SBG. 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.
8.1.1 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 8.2 on page 123 ).
• Use the Port Triggering screen to add and configure the SBG’s trigger port settings ( Section 8.3 on
page 126 ).
• Use the Address Mapping screen to configure the SBG's address mapping settings ( Section 8.4 on
page 129 ).
• Use the Default Server screen to configure a default server ( Section 8.5 on page 131 ).
• Use the ALG screen to enable and disable the NAT and SIP (VoIP) ALG in the SBG ( Section 8.6 on
page 133 ).
8.1.2 What You Need To Know
Inside/Outside
Inside/outside denotes where a host is located relative to the SBG, 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.