Subnet mask of IPs

[Spencer]

IPv4 addresses are divided into three sections: the network part, the subnet part, and the host part.

Following are the 5 classes,
Class A = 0.0.0.0 - 127.255.255.255/255.0.0.0
Class B = 128.0.0.0 – 191. 255.255.255/255.255.0.0
Class C = 192.0.0.0 – 223. 255.255.255/255.255.255.0
Class D = 224.0.0.0 – 239. 255.255.255
Class E = 240.0.0.0 – 255.255.255.255

127.0.0.0/8 network is in the Class A area, it’s loopback and cannot be assigned to a network.
Class D multicasting
Class E reserved
Subnetting is the method of allocating bits from the host bit as a network bit.
Each Octet is divided in 8 bits in the power of 2 as follows
Last octet in the range of N.N.N.H
7 6 5 4 3 2 1 0
128 64 32 16 8 4 2 1
Following diagram shows the distribution of the bits resulting into subnet mask.