What is Network Address Translation (NAT)?
The best way to describe how NAT work is to liken it to an extension of an office telephone line. An outside caller calls only the main number that connects to the office and the switchboard operator looks through the office telephone list and connects the caller to the particular office the call is meant for. The particular office could leave instruction with the receptionist or whomever works at the switchboard to forward or not to forward the call.
Unlike DHCP server that assigns IP dynamic addresses to devices inside the network, NAT-enabled routers retain one or many valid Internet IP addresses outside of the network. When the client sends packets out of the network, NAT translates the internal IP address of the client to an external address.
To outside users, all traffic coming to and going from the network has the same IP address or is from the same pool of addresses.
NAT has different functions, but its key function is to save IP addresses by allowing networks to use private IP addresses. NAT translates private, internal addresses into public, external addresses. NAT has an added benefit of adding a degree of privacy and security to a network because it hides internal IP addresses from outside networks.
The following terms are used when discussing NAT:
- Inside local address– Usually not an IP address assigned by a service provider and is most likely a private address.
- Inside global address– Valid Public IP address that the inside host is given when it exits the NAT configured router.
- Outside global address– Valid public IP address assigned to a host on the Internet.
- Outside local address– The local IP address assigned to a host on the outside network. In most situations, this address will be identical to the outside global address of that outside device.
To make it clearer, the address internal devices use to communicate with other internal devices is the inside local address.
The address internal devices use to communicate with external devices is the outside local address.
The address external devices uses to communicate with internal devices is the inside global address.
Finally, external devices communicate with one another using outside global addresses.
Summary of Scaling the Network with NAT and PAT
■ There are three types of NAT: static, dynamic, and overloading (PAT).
■ Static NAT is one-to-one address mapping. Dynamic NAT addresses are picked from
■ NAT overloading (PAT) allows you to map many inside addresses to one outside
■ Use the show ip nat translation command to display the translation table and verify
that translation has occurred.
■ To determine whether a current translation entry is being used, use the show ip nat
statistics or clear ip nat statistics commands to check and clear the hits counter.
■ Use the debug ip nat command to verify translation of packets.