ZyXEL Communications LTE5121, User Manual

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LTE-5121 User’s Guide
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Network Address Translation (NAT)
10.1 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.
10.1.1 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 10.2 on page 116 ).
• Use the DMZ screen to configure a default server ( Section 10.3 on page 119 ).
• Use the Sessions screen to limit the number of concurrent NAT sessions each client can use
( Section on page 118 ).
• Use the ALG screen to enable or disable the SIP ALG ( Section 10.5 on page 120 ).
10.1.2 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 LTE 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.