Classful Networking

Class	First Octet Range	Default Subnet Mask	Purpose
A	0–127	255.0.0.0 (/8)	Large networks (e.g., ISPs, big organizations)
B	128–191	255.255.0.0 (/16)	Medium-sized networks (e.g., enterprises, universities)
C	192–223	255.255.255.0 (/24)	Small networks (e.g., SMBs, home networks)
D	224–239	—	Multicast (group communication)
E	240–255	—	Reserved (experimental / future use)