ZyXEL Communications VSG1435-B101 - V1.10, Manual

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Chapter 12 Network Address Translation (NAT)
VSG1435-B101 Series User’s Guide
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Note that inside/outside refers to the location of a host, while global/local refers to
the IP address of a host used in a packet. Thus, an inside local address (ILA) is the
IP address of an inside host in a packet when the packet is still in the local
network, while an inside global address (IGA) 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 global) of an outside host.
12.8.2 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 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. Note that the
IP address (either local or global) of an outside host is never changed.
The global 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 (for Many-to-One and Many-to-
Many Overload mapping), 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. For more information on IP
address translation, refer to
RFC 1631 , The IP Network Address Translator (NAT) .
Table 51 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.
Global This refers to the packet address (source or destination) as the packet travels
on the WAN.