252 Chapter 7: Managing Address Spaces with NAT and IPv6
?– Conserves addresses through application port-level multiplexing. With NAT, internal
hosts can share a single registered IPv4 address for all external communications. In this
type of con?¬?guration, relatively few external addresses are required to support many
internal hosts, thus conserving IPv4 addresses.
?– Protects network security. Because private networks do not advertise their addresses or
internal topology, they remain reasonably secure when they gain controlled external
access in conjunction with NAT.
One of the main features of NAT is PAT, which is also referred to as ???overload??? in Cisco
IOS con?¬?guration. PAT allows you to translate multiple internal addresses into a single
external address, essentially allowing the internal addresses to share one external address.
Figure 7-2 shows an example of Port Address Translation. The following list highlights the
operations of PAT:
Figure 7-2 Port Address Translation
?– PAT uses unique source port numbers on the inside global IPv4 address to distinguish
between translations. Because the port number is encoded in 16 bits, the total number
of internal sessions that NAT can translate into one external address is, theoretically,
as many as 65,536.
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