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Network Address Port Translation (NAPT)
NAPT extends the concept of translation one step further by also trans-
lating transport identifiers—TCP and UDP port numbers—and ICMP query
identifiers. This allows the transport identifiers of a number of private
hosts to be multiplexed into the transport identifiers of a single global IP
address. NAPT allows numerous hosts from the inside network to share a
single outside network IP address. The advantage of this type of translation
is that only one global IP address is needed for every 6,400 simultanious
sessions, whereas with NAT, each inside host must translate to a unique
outside IP address.
TIP
Both NAT and NAPT can be combined; the advantage is that when NAT
exhausts the pool of global IP addresses, NAPT can then be used until
one of the NAT translations is timed out. This method ensures that all
inside hosts can be translated successfully into outside global IP
addresses.
Figure 3.4 illustrates NAPT. Host A on the inside network needs to
communicate with Host Z on the outside network. Because these two hosts
are on different networks and the inside network uses IP addresses from a
private address space, NAT/NAPT is needed to allow the two hosts to com-
municate. Unfortunately the administrator has only a limited number of
global IP addresses, many of which have already been assigned to various
devices. Therefore NAT cannot be used for translations.
As an alternative, NAPT can be used instead. To perform NAPT:
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