The Advantages of NAT

The Advantages of NAT
There are many advantages to using NAT. In this section, you will learn about some of the more
important benefits, including the following:

NAT allows you to incrementally increase or decrease the number of registered IP addresses
without changing devices (hosts, switches, routers, and so on) in the network. You still need
to change the device doing the NAT but not every other device.

NAT can be used either statically or dynamically:

Static translations are manually configured to translate a single local IP address to a
single global IP address, and vice versa. This translation always exists in the NAT table
until it is manually removed. Optionally, this translation could be configured between
a single local IP address and port pair to a single global IP address and port pair using
either TCP or UDP. These port values needn’t be the same value.

Dynamic mappings are configured on the NAT border router by using a pool of one or
more registered IP addresses. Devices on the inside network that wish to communicate
with a host on the outside network can use these addresses in the pool. This allows multiple
internal devices to utilize a single pool of IP addresses. You can go even further and
use a single IP address by configuring overloading, which will translate both the IP
address and port number.

NAT can be configured to allow the basic load sharing of packets among multiple servers
using the TCP load distribution feature. TCP load distribution uses a single virtual global
IP address, which is mapped to multiple real local IP addresses. Incoming connections are
distributed in a round-robin fashion among the IP addresses in the local pool. The packets
for each individual connection, or flow, are sent to the same local IP address to ensure
proper session communications.
There is no artificial limit to the number of NAT connections that can be active on
a router at any given time. The limit is determined by the amount of DRAM available
on the NAT router. Each NAT translation is stored in RAM and uses approximately
160 bytes. This means that about 1.53MB of RAM (often rounded to 1.6MB
in Cisco documentation) is required for 10,000 NAT translations, which is far more
than the average router needs to provide.

If you switch Internet service providers (ISPs) and need to change the registered IP addresses
you are using, NAT makes it so you don’t have to renumber every device in your network.
The only change is the addresses that are being used in the NAT pool.

NAT also helps if you have merged with another company and you’re both using the same
RFC 1918 address space. You can configure NAT on the border router between your routing
domains to translate the address from one network to the other, and vice versa, with
each side spoofed into believing the other side is in a different, non-conflicting network.