Look up the classful class and default subnet mask for any IPv4 address.
Class is decided by the first octet: A 0-127, B 128-191, C 192-223, D 224-239, E 240-255. Classes D and E have no default mask.
10.1.2.3 → Class A, default mask 255.0.0.0The mask implied by class alone: A=/8, B=/16, C=/24. Modern routing uses CIDR instead.
Class D is multicast and Class E is experimental, so neither has a classful host mask.