RFC 1918 IPv4 Addressing
Intro
Request For Comment (RFC) article1918 is a publication from the Internet Engineering Task Force (IETF) which has been adopted by the Internet Assigned Numbers Authority (IANA) as a standard, for private IPv4 address space.
https://tools.ietf.org/html/rfc1918
This private IPv4 address space is not publicly routable and therefore any organisation can use it 'behind the fence' or on their Local Area Network (LAN) without the need for approval from regional or internet registries.
Address Ranges
Three ranges are reserved for private IPv4 address use as shown above. Depending on the number of subnets/hosts required within your private network will depend on which range you choose. Utilising these reserved ranges alleviates exhaustion of the public IPv4 availability.