There are many advantages of using NAT. Some of the more important benefits include the
following:
NAT enables you to incrementally increase or decrease registered IP addresses without
changes to hosts, switches, or routers within the network. (The exception to this is the NAT
border routers that connect the inside and outside networks.)
NAT can be used either statically or dynamically:
Static translation occurs when you manually configure an address translation table with
IP addresses. A specific address on the inside of the network uses a specific outside IP
address—manually configured by the network administrator—to access the outside network.
The network administrator can also translate an inside IP address and port pair
to an outside IP address and port pair.
Dynamic mappings enable the administrator to configure one or more pools of outside
IP addresses on the NAT border router. The addresses in the pools can be used by nodes
on the inside network to access nodes on the outside network. This enables multiple
internal hosts to utilize a single pool of IP addresses.
NAT can allow the sharing of packet processing among multiple servers by using the Transmission
Control Protocol (TCP) load distribution feature. NAT load distribution can be
accomplished by using one individual global address mapped to multiple local server
addresses. This round-robin approach is used on the router distributing incoming connections
across the servers.
There is no limit to the number of NAT sessions that can be used on a router or
route processor. The limit is placed on the amount of DRAM the router contains.
The DRAM must store the configurable NAT pools and handle each translation.
Each NAT translation uses approximately 160 bytes, which translates
into about 1.53MB for 10,000 translations. This is far more translations than the
average router needs to provide.
If your internal addresses must change because you have changed your ISP or have merged
with another company that is using the same address space, you can use NAT to translate
the addresses from one network to the other.