3Com 3C13636 Configuration Manual page 514

3Com Router 3000 Ethernet Family
Configuration Guide
Note:
The number of public IP addresses on the NAT server is far less than the number of
hosts in the intranet because not all hosts will access the extranet at one time. The
public IP address number is determined based on the maximum number of intranet
hosts at the rush hour of the network.
In practice, it may be required that only some intranet hosts can access the Internet
(external network). In other words, the NAT server will not translate source IP
addresses of those unauthorized hosts, which is called address translation control.
Router implements many-to-many address translation and address translation control
via address pool and ACL respectively.
Address pool: A set of public IP addresses for address translation. A client should
configure an appropriate address pool according to its valid IP address number,
internal host number as well as the actual condition. An address will be selected
from the pool as the source address during the translation process.
ACL-based address translation: Only the data packet matching the ACL rule can
be translated, which effectively limits the address translation range and allows
some specific hosts to access Internet.
11.2.2 NAPT
There is another way to implement the concurrent access, that is, Network Address
Port Translation (NAPT), which allows the map from multiple internal addresses to an
identical public address. Therefore, it can be called as "many-to-one address
translation" or address multiplex informally.
NAPT maps IP addresses and port numbers of data packets form various internal
addresses to an identical public address with different port numbers. In this way,
different internal addresses can share an identical public address.
The fundamentals of NAPT are shown in the following figure.
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11-3
Chapter 11 NAT Configuration