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Chapter 10 Network Address Translation (NAT)
P-660RU-Tx v3 User’s Guide
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Note that inside/outside refers to the location of a host, while public/local refers to the IP
address of a host used in a packet. Thus, an inside local address is the IP address of an inside
host in a packet 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
ZyXEL Device 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 ZyXEL Device 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.