Sent from my iPhone
Recently, I was made aware that a class "A" was indeed a /8 and a class "B" was actually a /12 (172.16/
172.31.255.255) while a class "C" is actually a /16.
You had it right to start with.
A is (was) /8, B is /16, C is /24
All on human easily readable byte boundaries in IPv4 space.
The RFC-1918 internal space was allocated from a /8, a /12, and a /16 sized block. Those aren't A, B, or C network sizes. Whoever corrected you is confused.
Anyone who networked before and during the CIDR transition won't forget this...
-george