A well-known RFC used to reserve a portion of IPv4 address space for private networks is RFC 1918, Address Allocation for Private Internets. Private IPv4 addresses are addresses that are not routed by Internet routers.

The developers of the ARPANET and TCP/IP never envisioned a world where an average person would have multiple computing devices (desktop computer, laptop, smartphone, tablet computer, etc.) all interconnected to a global Internet. Even in the late 1970s personal computers were only of interest to a small minority of computer hobbyists and the idea of networking them was still years away. In 1981 when the current IPv4 RFC was published (RFC 791, Internet Protocol), a 32-bit IPv4 address with a theoretical possibility of just over four billion addresses seemed more than adequate.

However, by the early 1990s with innovations in personal computing, email and the World Wide Web, the number of devices accessing the Internet was growing rapidly. By the mid-1990s it was obvious that networks would soon be running out of IPv4 addresses. Additional address space was needed that any organization could use to address devices with the caveat that these addresses could not be used for accessing the Internet. These addresses became known “private IPv4 addresses” and standardized in 1996 with RFC 1918. If you have a home router you are more than likely using one or more of these private IPv4 addresses. Combined with another technology known as network address translation (NAT), devices using private IP addresses are able to access the Internet through a process where the private IPv4 address is converted to a public IPv4 address.

Private addresses are also known as RFC 1918 addresses. RFC 1918 discusses the motivation behind private addressing and specifies the three ranges of addresses reserved as private IPv4 addresses. As indicated in RFC 1918, the IANA has reserved the following three blocks of IPv4 address space for private networks: