What is the IPv4 'Class A, B, C' classful addressing system, and what were the historical subnet masks for each class?

• A.Class A = /16, Class B = /24, Class C = /32.
• B.Classful addressing is still used today; CIDR notation is only used in documentation.
• C.Classful addressing (pre-CIDR): Class A = first bit 0, range 1.0.0.0-126.0.0.0, default mask /8 (255.0.0.0), 16 million hosts per network; Class B = first bits 10, range 128.0.0.0-191.255.0.0, default mask /16 (255.255.0.0), 65534 hosts; Class C = first bits 110, range 192.0.0.0-223.255.255.0, default mask /24 (255.255.255.0), 254 hosts. CIDR (1993) replaced classful addressing with variable-length masks.
• D.Class A = ranges 10.x.x.x (private only), Class B = 172.16-31.x.x, Class C = 192.168.x.x. The 100.64.0.0/10 shared address space is the IPv4 multicast test range. The 172.16.0.0/12 block is public space routable on the internet without translation. DHCP assigns the same address to every host in a subnet, distinguishing them by port. The ping utility uses TCP handshakes to measure round-trip time to a host