Before CIDR, addresses were divided into classes. Class A used the first octet for network (-), leaving bits for hosts. Class B used two octets for network (-). Class C used three octets (-). This was wasteful. A company needing hosts got a Class B with addresses. CIDR replaced this rigid system, but you'll still hear these terms.
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
$ curl repovive.com/roadmaps/network-design/ip-addressing-and-subnetting/historical-address-classes
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