Private IPv4 and IPv6 address spaces

In the Internet addressing architecture, a private network is a network that uses private IP address space, following the standards set by RFC 1918 for Internet Protocol Version 4 (IPv4), and RFC 4193 for Internet Protocol Version 6 (IPv6). These addresses are commonly used for home, office, and enterprise local area networks (LANs), when globally routable addresses are not mandatory, or are not available for the intended network applications. Under IPv4, the private IP address spaces were originally defined in an effort to delay IPv4 address exhaustion, but they are also a feature of IPv6, the next generation Internet Protocol.

These addresses are characterized as private because they are not globally delegated, meaning that they are not allocated to any specific organization, and IP packets addressed with them cannot be transmitted through the public Internet.

Private IPv4 address spaces:

The Internet Engineering Task Force (IETF) [1] has directed the Internet Assigned Numbers Authority (IANA) [2] to reserve the following IPv4 address ranges for private networks, as published in RFC 1918 [3].

RFC1918 name	IP address range	number of addresses	largest CIDR block (subnet mask)	host id size	mask bits	classful description
24-bit block	10.0.0.0 - 10.255.255.255	16,777,216	10.0.0.0/8 (255.0.0.0)	24 bits	8 bits	Single class A network
20-bit block	172.16.0.0 - 172.32.255.255	1,048,576	172.16.0.0/12 (255.240.0.0)	20 bits	12 bits	16 contiguous class B networks
16-bit block	192.168.0.0 - 192.168.255.255	65,536	192.168.0.0/16 (255.255.0.0)	16 bits	16 bits	256 contiguous class C networks

Dedicated space for Carrier Grade NAT deployments:
In April 2012, IANA allocated 100.64.0.0/10 for use in carrier grade NAT [4] scenarios in RFC 6598 [5]. This address block should not be used either on private networks or on the public Internet: it is intended only for use within the internal operations of carrier networks. 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 the Tokyo metropolitan area.

Private IPv6 address spaces:
The concept of private networks and special address reservation for such networks has been carried over to the next generation of the Internet Protocol, IPv6.

The address block fc00::/7 has been reserved by IANA as described in RFC 4193 [6]. These addresses are called Unique Local Addresses (ULA). They are defined as being unicast in character and 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.

A former standard proposed the use of so-called "site-local" addresses in the fec0::/10 range, but due to major concerns about scalability and the poor definition of what constitutes a site, its use has been deprecated since September 2004 by RFC 3879.

[1] http://www.ietf.org
[2] http://www.iana.org
[3] http://tools.ietf.org/html/rfc1918
[4] http://en.wikipedia.org/wiki/Carrier-grade_NAT
[5] http://tools.ietf.org/html/rfc6598
[6] http://tools.ietf.org/html/rfc4193