EE5412 Telecommunications Network NAT

EE5412 NAT, Network Address Translation

Network address translation (NAT) allows a site to use a set of private addresses for internal communication and a set of global Internet addresses for communication with external site.

The site must have only one single connection to the global Internet through a router that runs NAT software.

The private network uses private addresses. The router that connects the network to the global address uses one private address and one global address. The private network is transparent to the rest of the Internet. The rest of the internet sees only the NAT router with its own global IP.

Translation table - The NAT router keeps a translation table co-relating the private address with the external Internet destination address.

All out going packets go through the NAT router. The source address in the outgoing packets will be replaced with the global NAT address.

All incoming packets also pass through the NAT router. The destination address of the incoming packets will be replaced with the appropriate private address. The private host address is identified by the external host address on the translation table.

With this technique, communication must always be initiated by the private network so that the NAT table can be updated before communication with outside internet.
A private network can not run a server program for clients outside of its network if it is using NAT technology.






Pool of IP address

Allow multiple private network hosts to access the same external host at the same time. Private host address can be identified by the external host address and the NAT router IP address.

IP Address and Port Address
To allow many-to-many relationship between private-network host and external server programs, private port address can also be used.