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ZyXEL AMG1001-T Series Manual

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Chapter 10 Network Address Translation (NAT)
AMG1001-T/AMG1011-T Series User’s Guide
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when the packet is still in the local network, while an inside public address is the IP address of the
same inside host when the packet is on the WAN side. The following table summarizes this
information.
NAT never changes the IP address (either local or public) of an outside host.
10.6.2 What NAT Does
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 public address) before forwarding the packet to the
WAN side. When the response comes back, NAT translates the destination address (the inside
public address) back to the inside local address before forwarding it to the original inside host. Note
that the IP address (either local or public) of an outside host is never changed.
The public IP addresses for the inside hosts can be either static or dynamically assigned by the ISP.
In addition, you can designate servers, for example, a web server and a telnet server, on your local
network and make them accessible to the outside world. If you do not define any servers, NAT
offers the additional benefit of firewall protection. With no servers defined, your AMG1001-T/
AMG1011-T filters out all incoming inquiries, thus preventing intruders from probing your network.
10.6.3 How NAT Works
Each packet has two addresses – a source address and a destination address. For outgoing packets,
the inside local address is the source address on the LAN, and the inside public address is the
source address on the WAN. For incoming packets, the inside local address is the destination
address on the LAN, and the inside public address 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 AMG1001-T/AMG1011-T keeps track of the original addresses and port numbers so
incoming reply packets can have their original values restored. The following figure illustrates this.
Table 26 NAT Definitions
ITEM DESCRIPTION
Inside This refers to the host on the LAN.
Outside This refers to the host on the WAN.
Local This refers to the packet address (source or destination) as the packet travels on the LAN.
Public This refers to the packet address (source or destination) as the packet travels on the WAN.
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