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from probing your network. For more information on IP address translation, refer to
RFC 1631, The IP Network Address Translator (NAT).
Inside/outside indicates where a host is located relative to the ROUTER. The
computers hosts of your LAN are inside, while the Web servers on the Internet are
outside.
Global/local indicates the IP address of a host in a packet as the packet traverses a
router. The local address refers to the IP address of a host when the packet is in the local
network, while the global address refers to the IP address of the host when the same
packet is traveling in the WAN side.
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 of 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.
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.
How NAT Works
Each packet has two addresses – a source address and a destination address. For
outgoing packets, the ILA is the source address on the LAN, and the IGA is the source
address on the WAN. For incoming packets, the ILA is the destination address on the
LAN, and the IGA 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 ROUTER keeps track of the original addresses
and port numbers so incoming reply packets can have their original values restored.
The following figure illustrates this.