With 24 bits on each network for distinguishing hosts, a Class A network represents a comparatively large community of potential IP addresses. Today, it is hard to qualify for even Class B network numbers. Sometimes an applicant organization is given a range of contiguous class C addresses. These days, it is more often an Internet Service Provider that makes application for networks. After the bits that identify the network part of the IP address, the rest of bits are completely at the discretion of the organization.
Most organizations have many more networks than registered network numbers.
In fact, many organizations have exactly one registered network number. Outside the organization, this subnet structure is not visible or important. Within the organization, the routers have to be told which bits are network bits beyond the boundary defined by the registered Class A, B, C rules. This is done by requiring a subnet mask whenever an IP address is assigned to a network device. If a position in the mask has a 1 bit, the corresponding bit of the IP address is a network bit.