RFC 3330, "Special-Use IPv4 Addresses," describes IPv4 address blocks that have been assigned by the Internet Assigned Numbers Authority (IANA) for specialized purposes, and includes reference to the private addresses defined in RFC 1918.
The remaining Class A, B, and C addresses are public addresses. Private addresses are for use only within a company's network; public addresses must be used when communicating on the public Internet. Internal private addresses must be translated to public addresses when data is sent out to the Internet, and these public addresses must be translated back to the private addresses when packets come in from the Internet.
Because only a finite number of public addresses are available, they are becoming scarce. Using private addresses internally on your network means that you will require fewer public addresses. However, public addresses are required for the Internet connections and for servers that must be accessible from the Internetfor example, File Transfer Protocol (FTP) servers that contain publicly accessible data, and web servers. Other devices internal to the network can use private addressesthey can connect to the Internet through a NAT device.
RFC 1631, "The IP Network Address Translator," defines NAT. NAT can be provided by a variety of devices, including routers and firewalls.
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