Work out ip address class instantly with clear inputs, formula shown and shareable results.
Classful addressing read the leading bits of the first octet: 0 meant class A with an 8-bit network part, 10 meant class B with 16 bits, 110 meant class C with 24 bits, 1110 marked multicast and 1111 was reserved. CIDR replaced this in 1993, but the class boundaries still explain default masks in older gear, certification questions and the shape of legacy allocations.
Class from first octet
1-126 = A (/8), 128-191 = B (/16), 192-223 = C (/24), 224-239 = D multicast, 240-255 = E reserved; hosts = 2^(32-prefix) - 2
Only for legacy defaults and exams. Modern routing is classless, so the mask is carried explicitly and any prefix length is valid regardless of the first octet.
127.0.0.0/8 is reserved for loopback, which is why the usable class A range stops at 126.