RFC 1918 is a document published by the Internet Engineering Task Force (IETF) that defines private IP address ranges. These addresses are not routable on the public internet, meaning they can be used within local networks without conflicting with public IP addresses. The specified ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The use of private IP addresses helps conserve the limited number of available public IP addresses. Organizations can create their own internal networks using these private addresses, allowing multiple devices to communicate without needing a unique public IP for each one.