Figure 18-4 Port Address Translation
Some of the benefits of NAT are as follows:
Reduced costs for renting public IPv4 addresses: With dynamic
NAT and especially with PAT, a large number of private internal
endpoints can be hidden behind a much smaller number of public IP
addresses. Since public IPv4 addresses have become a rare commodity,
there is a price for each public IPv4 address used. Large cost savings are
possible by using a smaller number of public addresses.
Conserving public IPv4 address space: The Internet would not
have witnessed the exponential growth of the past few decades without
NAT. Extensively reusing RFC 1918 private IP addresses for internal
networks helped slow the depletion of IPv4 address space.
Additional security due to hiding the addressing for internal
networks: Having a whole network hidden behind one or a pool of
public IPv4 addresses thwarts network reconnaissance attacks and
increases the defense capabilities of the network.
Flexibility and cost savings when changing ISP connectivity:
In cases in which external network connectivity needs to be changed
and migrations to new ISPs are required, configuration changes need to
be performed only on the network border devices, but the rest of the
internal network does not need to be re-addressed. This results in
massive cost savings, especially in large enterprise networks.Although the advantages of NAT, far outweigh the
disadvantages in most cases, there are some
disadvantages, including the following:
Loss of end-to-end functionality: Some applications, especially for
real-time voice and video signaling, are sensitive to changes in IP
header addressing. While establishing these types of real-time voice
and video sessions, headers exchanged at the application layer contain
information pertaining to the private non-globally routable IP
addresses of the endpoints. When using NAT in these cases, the IP