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Chapter 11 Network Address Translation (NAT)
VMG1312-B10A 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.
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.
11.8.2 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 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 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).
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 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 61
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.
LAN.
Global
This refers to the packet address (source or destination) as the packet travels on the
WAN.
WAN.