IP addressing and subnetting

In the early 1980s an IPv4 address was split by class: a university given a class B network had 16 bits of host space, more than 65,000 addresses. A campus with many LANs had two bad options: run one huge flat network, where every broadcast reached every host, or ask for a separate network number for each LAN and make every router on the internet carry them all.

In August 1985 Jeffrey Mogul and Jon Postel published RFC 950, the Internet Standard Subnetting Procedure. It let a site borrow bits from the host part to number its own internal networks, using a subnet mask that only routers inside the site needed to know. Every subnetting exercise you will ever do is a descendant of that idea.