Identify the range of IPv4 addresses spanned by Class A, Class B, and Class C

The range of IPv4 addresses spanned by each class is:

 Class A: 1.0.0.0 to 127.255.255.255
 Class B: 128.0.0.0 to 191.255.255.255
 Class C: 192.0.0.0 to 223.255.255.255 

A small organisation has a class c addresses for 7 network, each with 24 hosts, what is an appropriate subnet mask?

A Class C address requires 21 bits for its network ID, leaving 8 bits for the host ID and subnet ID to share. One possible scheme would assign 4bits to the host and 4 to the subnet ID, as shown below
Network-id : 0- 23
Subnet-id    : 24 -27
Host-id       :  28 31



Subnet mask: 255.255.255.224