RFC 1918 (Request for Comment 1918), titled “Address Allocation for Private Internets,” is an Internet Engineering Task Force (IETF) memorandum detailing the methods for assigning private IP addresses within TCP/IP networks.
RFC 1918, in conjunction with NAT (Network Address Translation), addresses the issue of IPv4 address exhaustion by expanding the usable number of IP addresses. This memorandum serves as a temporary solution to manage IP address scarcity until the adoption of IPv6. Unlike public IP addresses, private IPs do not need to be registered with a Regional Internet Registry (RIR), simplifying the setup of private networks.
RFC 1918 established the standards for how networking equipment assigns IP addresses in private networks. A private network can use a single public IP address, and the RFC designates specific IP address ranges that are not routable on the Internet:
Class A: 10.0.0.0 - 10.255.255.255 (10/8 prefix, common mask 255.0.0.0)
Class B: 172.16.0.0 - 172.31.255.255 (172.16/12 prefix, common mask 255.255.0.0)
Class C: 192.168.0.0 - 192.168.255.255 (192.168/16 prefix, common mask 255.255.255.0)
IP addresses within these ranges can be used within a private network, ensuring uniqueness within that network but not beyond it. As private IP addresses are not globally unique, external computers cannot directly communicate with them on the public Internet.
Within the private network, devices can access the Internet through NAT. NAT translates internal network IP addresses to different IP addresses known within another network. Typically, this involves mapping local network addresses to one or more global IP addresses for outgoing traffic, and vice versa for incoming traffic.
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