Short Note: NAT

Mumbai University > Electronics Engineering > Sem 8 > Advanced Networking Technologies

Marks: 5M

Year: Dec 2014

1 Answer
  1. A technology that can provide mapping between the private and universal addresses and at the same time support virtual private networks is network address translation (NAT). The technology allows a site to use a set of private addresses for internal communication and a set of global internet addresses for communication with rest of the world. The site must have only one single connection to the global internet through a NAT capable router that runs NAT software.
  2. Consider an office scenario where multiple computers are there where all computers are not required to be connected to the internet all the time. Some of those computers are used for other works and for communication among themselves. In such case office network is provided with one global address to connect to internet while communicating among themselves they use private address.

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  1. As the figure shows, the private network uses private addresses. The router that connects the network to global address uses one private address and one global address. This makes private network invisible to the rest of the internet as rest of the internet sees only NAT router with its global address of
  2. All of the outgoing packets go through the NAT router, which replaces the source addresses in the packet with the global NAT address. All incoming packets also pass through the NAT router, which replaces the destination address in the packet with appropriate private address.
  3. Translating the source address for outgoing packet is straight forward because all the source addresses are replaced with single NAT address. But it becomes difficult for router to know exactly which station does the received packet belongs to as there can be hundreds of stations inside private network.
  4. To do this translation table is maintained by the router which records destination address and source private address. Thus when packet received by the router from recorded destination address it is forwarded to respective source address from which communication was initiated for that destination address.
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