Subnetting, documented in RFC 950 | Software | Online Subnet Calculators
Online Subnet calculator
Supernet calculator (CIDR / VLSM)
Wildcard mask calculator

Subnetting allows you to create multiple logical networks that exist within
a single Class A, B, or C network. If you don't subnet, you will only be
able to use one network from your Class A, B, or C network. Unless you have
been assigned many major networks, you really need to subnet.

A subnet mask is defined for each IP address. The subnet mask identifies
which portion of the 4 octets is used to identify the data link, with the
remaining bits identifying the node. If you want no subnetting, use these
default masks (255 - strictly follow number, 0 - wildcard):

Class A: Ip Range starting with : 1-126
Class B: Ip Range : 128-191
Class C: Ip Range : 192-254