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Overlapping Networks 339
The overloading feature of NAT uses the entire socket to track the sender; thus, the same IP
address can be substituted for many sending addresses, as illustrated in Figure 11-4.
Figure 11-4
Overloading of Substitute Addresses
Each device that is sending through the NAT device in Figure 11-4 is translated and given a new
socket number. The new socket number has a unique port number known by the router or NAT
device and a common IP address for each translation. In this fashion, only one legitimate
address is required for the translation. The use of the port to make the translation unique is
called PAT. With PAT, the entire socket is replaced.
Overlapping Networks
Another use of NAT occurs when two networks are overlapped, or using the same numbering
scheme. If they are merged, the IP address scheme fails because of the overlap. This NAT
function is not something that should be designed into a network.
NAT overlap aids the administrator when a merger occurs. The two entities, without the
renumbering of each end station, can be consolidated. In this fashion, the administrator can
focus on putting a renumbering plan in place.
Translation Table:
10.1.1.1:1456 133.4.5.6:1456
10.1.1.2:1D48 133.4.5.6:1897
10.1.1.3:1776 133.4.5.6:1999
10.1.1.1
Source Port
1456
10.1.1.2
Source Port
1D48
10.1.1.3
Source Port
1776
Address Pool:
133.4.5.6­133.4.5.6
NAT
Device