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CCENT™ (Cisco Certified Entry Network Technician) Certification Exam Notes

G. NAT

1. Given below are the four important forms of NAT (Network Address Translation):

1. Static NAT: It is a one-to-one mapping between an unregistered IP address and a registered IP address.
2. Dynamic NAT: Usually, Dynamic NAT is implemented, where a pool of public IP addresses is shared by an entire private IP subnet. When a private host initiates a connection, a public IP address is selected. The mapping of the computer’s non-routable IP address matched to the selected IP address is stored in the NAT Table. As long as the outgoing connection is maintained, the private host can be reached by incoming packets sent to the specified public address. When the binding expires, the address is returned to the pool for reuse.
3. Overloading: A variation of Dynamic NAT, also known as Network Address Port Translation (NAPT) maps multiple unregistered IP addresses to a single registered IP address by multiplexing streams differentiated by the TCP/UDP port number.
4. Overlapping: When Overlapping is employed, the IP addresses used on the internal network are registered IP addresses utilized on another network. To avoid conflict, a NAT Table is built to translate these redundant internal addresses to a unique IP address. Vice versa, when sending packets into the private network, the registered addresses must be translated to an address unique in the network.

2. Important Network Address Translation (NAT) terms of inside local, inside global, outside local, and outside global are explained below:

a. Inside local: A private IP address assigned to a host on the inside network.
b. Inside global: A public IP address that represents one or more inside local IP addresses to the outside world.
c. Outside local: The IP address of an outside host as it appears to the inside network. Not necessarily a legitimate address, it is allocated from an address space routable on the inside.
d. Outside global: The IP address assigned to a host on the outside network. The address is allocated from a globally routable address or network space.

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