Summary
In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Both the IPv4 and the IPv6 specifications define private IP address ranges. Private network addresses are not allocated to any specific organization. Anyone may use these addresses without approval from regional or local Internet registries. Private IP address spaces were originally defined to assist in delaying IPv4 address exhaustion. IP packets originating from or addressed to a private IP address cannot be routed through the public Internet. The Internet Engineering Task Force (IETF) has directed the Internet Assigned Numbers Authority (IANA) to reserve the following IPv4 address ranges for private networks: In practice, it is common to subdivide these ranges into smaller subnets. IPv4 shared address space In April 2012, IANA allocated the block 100.64.0.0/10 (100.64.0.0 to 100.127.255.255, netmask 255.192.0.0) for use in carrier-grade NAT scenarios. This address block should not be used on private networks or on the public Internet. The size of the address block (222, approximately 4 million addresses) was selected to be large enough to uniquely number all customer access devices for all of a single operator's points of presence in a large metropolitan area such as Tokyo. Unique local address The concept of private networks has been extended in the next generation of the Internet Protocol, IPv6, and special address blocks are reserved. The address block fc00:: is reserved by IANA for unique local addresses (ULAs). They are unicast addresses, but contain a 40-bit random number in the routing prefix to prevent collisions when two private networks are interconnected. Despite being inherently local in usage, the IPv6 address scope of unique local addresses is global. The first block defined is fd00::, designed for /48 routing blocks, in which users can create multiple subnets, as needed.
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