Public and Private IP Addresses
Internet stability depends directly on the uniqueness of publicly used network addresses. Therefore, some mechanism is needed to ensure that addresses are, in fact, unique. Originally, it was the responsibility of an organization that is known as the InterNIC. IANA succeeded the InterNIC. IANA carefully manages the remaining supply of IP addresses to ensure that duplication of publicly used addresses does not occur. Such duplication would cause instability in the Internet and would compromise its capability to deliver datagrams to networks using the duplicated addresses.
To obtain a public (globally unique) IP address or a block of public IP addresses, you must contact an ISP. The ISP then contacts the upstream registry or the appropriate regional registry at one of these organizations:
- RIPE NCC
While Internet hosts require a globally unique IP address, private hosts that are not connected to the Internet can use any valid address. However, these addresses must be unique within the private network. But because many private networks exist alongside public networks, grabbing “just any address” is strongly discouraged. RFC 1918 specifies a set of IP addresses that is reserved for private networks.
These addresses are not routed on the Internet backbone. When a network using private addresses must connect to the Internet, it is necessary to translate the private addresses to public addresses. This translation process is known as NAT. A router is often the network device that performs NAT. The tables below show the private and public IP address ranges.