Chapter 10 Network Address Translation (NAT)
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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.
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.
NAT never changes the IP address (either local or global) 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 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, 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)
.
