For better understanding: The subnet mask is used to divide an IP address into two logical sub items, The subnet address and the host address. The common subnet mask of 255.255.255.0 indicates that for the the 32 bits of IP address the leftmost 24 bits are the subnet address portion and the rightmost 8 bits are the Host address portion.

In this case there can be up to 256 host address but a few of them (such as 255 and 0) are reserved.

For hosts to be on the same subnet the subnet portions of their IP address (as defined by the subnet mask) must be the same. For example, hosts with IP addresses 192.168.1.35 and 192.168.1.56 are on the same subnet assuming a subnet mask of 255.255.255.0; while 192.168.1.35 and 192.168.2.55 are not on the same subnet assuming the same subnet mask.