What is the Difference Between NAT and PAT?
By Stephen Bucaro
The main purpose for NAT (Network Address Translation) is to hide internal network
IP addresses from the external Internet for security reasons. It works by mapping
a pool of public IP addresses one-to-one to the private internal IP addresses,
keeping the port number intact.
| Internal Address | External Address |
| 10.10.10.1:80 | 63.63.63.1:8000 |
| 10.10.10.2:80 | 63.63.63.1:8001 |
| 10.10.10.3:80 | 63.63.63.1:8002 |
| 10.10.10.4:80 | 63.63.63.1:8003 |
A port number identifies a specific protocol or process on a host to which a message
or datagram is to be delivered. PAT (Port Address Translation) uses a single public
IP address and maps multiple inside addresses to it by using different port numbers.
Most routers actually use NAPT (Network Address and Port Translation), which translates
not only IP addresses, but also TCP and UDP port numbers.
An additional advantage of NAT is that by allowing multiple computers to connect to
the Internet through a single external IP address it reduces the cost for connecting
to the Internet. It also helped to extend the life of dwindling number of IPv4 addresses
until IPv6 could be implemented.
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