RFC 1918, also known as Request for Comment 1918, is the Internet Engineering Task Force (IETF) record on methods of assigning private IP addresses on TCP/IP networks. RFC 1918 outlines the usable private IP addresses available under IPv4. It’s not necessary to register private IPs with a Regional Internet Registry (RIR), which simplifies setting up private networks.
RFC 1918 was used to create the standards where networking equipment assigns IP addresses in a private network. A private network can use a single public IP address. For example, all broadband modems perform Network Address Translation (NAT) converting their single Internet Routable IP Address (a public IP) to one of these RFC1918 IP addresses (a private IP address). This is done automatically for all devices on an internal network providing many IP addresses to one public IP address conversions and keeping track in a database on the Modem. The RFC reserves the following ranges of IP addresses that cannot be routed on the Internet:
- 10.0.0.0 – 10.255.255.255 (10/8 prefix)
- 172.16.0.0 – 172.31.255.255 (172.16/12 prefix)
- 192.168.0.0 – 192.168.255.255 (192.168/16 prefix)
IP addresses within these ranges can be assigned within a private network; each address will be unique on that network but not outside of it. Private IP addresses can’t be communicated directly by external computers because they are not globally unique and not addressable on the public Internet.