
Configuring Network Address Translation
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12-3
For example, a host sends an outbound packet from inside company A to company
B. The packet follows normal IP routing to the NAT border router at the egress
point in company A. When the NAT interface receives the packet, NAT software
extracts the source address and compares it to an internal table of existing address
translations. If the inside host’s source address does not appear in the translation
table, NAT software does the following:
1.
Creates a new entry for the host
2.
Assigns a globally unique IP number dynamically from a pool of available
addresses
3.
Changes the source address of the packet to the globally unique address
The router software then forwards the packet through the Internet to the NAT
border router in company B.
When the packet arrives at company B, router software routes the packet to the
destination local address within company B.
After a specified timeout period during which there have been no translated
packets for a particular address translation, NAT software within company A
removes the entry, freeing the global address for use by another inside host.
In Figure 12-1
, a packet from company A’s network with unregistered source
address 10.0.0.15 is sent to a destination address in company B’s network. The
destination is a globally recognized registered address, 192.100.20.2.
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