IPv4 Address Classes & Subnet Masks

This image shows IPv4 Address Classes & Subnet Masks
Subnet Mask decides which part of the IP is the Network ID and which part is the Host ID.
Black part = Network portion
Yellow part = Host portion
Breakdown:

Class A​

Subnet Mask: 255. 0. 0. 0
Network = 8 bits
Host = 24 bits
Used for very large networks (millions of hosts)

Class B​

Subnet Mask: 255. 255. 0. 0
Network = 16 bits
Host = 16 bits
Used for medium-sized networks

Class C​

Subnet Mask: 255. 255. 255. 0
Network = 24 bits
Host = 8 bits
Used for small networks (up to 254 hosts)
Loopback (127. 0. 0. 1)
Subnet Mask: 255. 255. 255. 255
Used to test your own computer

So:
- Class A → Big networks
- Class B → Medium networks
- Class C → Small networks
- Loopback → Self test