Every networking course begins with the same piece of history: class a b c d e networks, the original system for dividing up IPv4 addresses. Before subnets and CIDR notation existed, an IP address carried its own size label in its first few bits, and the class told routers and engineers how much of the address named the network and how much named the host. This guide explains class a b c d e networks one class at a time, with their ranges and purposes, and then shows why the system was replaced and where its fingerprints still appear.
The classful system is officially retired, but it refuses to leave. Terms like “a class C network” are still used daily by working network engineers as shorthand, and the default subnet masks of the old classes still surface in equipment and exams. Learning class a b c d e networks properly is the fastest route to understanding both.
How class a b c d e networks divided the address space
An IPv4 address is 32 bits long, written as four decimal numbers. In the classful design, the first bits of the address announced its class, and the class fixed the boundary between the network portion and the host portion. Class A gave a few networks enormous numbers of hosts. Class B split the difference. Class C gave many networks a modest number of hosts each. Classes D and E were set aside for special purposes rather than ordinary addressing. That simple scheme is the whole idea behind class a b c d e networks: five buckets, each with a defined range of first-octet values, a default mask and an intended job.
The ranges follow from the leading bits. Class A addresses begin with a 0 bit, giving first octets from 0 to 127. Class B begins with 10, giving 128 to 191. Class C begins with 110, giving 192 to 223. Class D begins with 1110, giving 224 to 239, and Class E begins with 1111, covering 240 to 255. Once you see the binary pattern, the ranges of class a b c d e networks stop being numbers to memorise and become a pattern you can rebuild on demand.
| Class | First octet range | Default mask | Purpose |
|---|---|---|---|
| A | 0 to 127 | 255.0.0.0 | Very large networks; few networks, millions of hosts |
| B | 128 to 191 | 255.255.0.0 | Medium to large organisations; thousands of hosts |
| C | 192 to 223 | 255.255.255.0 | Small networks; up to 254 hosts |
| D | 224 to 239 | Not applicable | Multicast: one stream delivered to many receivers |
| E | 240 to 255 | Not applicable | Experimental and reserved; not for general use |
Classes A, B and C: the working networks
Class A was built for giants. With the network portion only 8 bits long, there were barely more than a hundred possible class A networks, but each could hold over sixteen million hosts. They went to very large organisations, major internet providers and government bodies. One range inside class A has a special job: addresses beginning with 127 are loopback addresses, used by a computer to talk to itself, with 127.0.0.1 the famous example. The class a b c d e networks scheme reserved it almost as an afterthought, and it is now among the most used addresses on Earth.
Class B balanced the split at 16 bits each for network and host, giving over sixteen thousand networks of about sixty-five thousand hosts, the natural size for universities and large companies. Class C multiplied the networks into the millions by shrinking the host portion to 8 bits, 254 usable hosts, which suited the small offices and departments that made up most real networks. In practice, class C allocations became the everyday currency of the early commercial internet, and its default mask, 255.255.255.0, remains the one home and small-office users meet most often, a direct inheritance from class a b c d e networks.
Classes D and E: multicast and the reserved shelf
Class D broke the pattern, because it was never meant to address individual machines. Addresses from 224 to 239 are multicast groups: a sender transmits to the group address, and the network delivers copies to every subscribed receiver. Multicast underpins some streaming, conferencing and service-discovery protocols. When students ask why class a b c d e networks include a class with no network and host split at all, the answer is that the designers were reserving tools for jobs beyond one-to-one communication.
Class E, covering 240 to 255, was reserved for experimental use and never released for general addressing. One address in it is famous anyway: 255.255.255.255, the limited broadcast address, used to reach every host on the local network. The rest of class E sits unused, a reminder that the designers of class a b c d e networks tried to leave room for a future that arrived in a different shape than they expected.
Why CIDR replaced the class system
The classful system wasted addresses on a heroic scale. An organisation needing three hundred hosts was too big for class C and received a class B, leaving tens of thousands of addresses idle, while the supply of unallocated networks ran down alarmingly fast in the early 1990s. The fix, introduced in 1993, was Classless Inter-Domain Routing, CIDR, which threw away the fixed boundaries of class a b c d e networks and let the network portion be any length, written with the familiar slash notation such as /24. Address blocks could then be sized to actual need, and routing tables could be summarised efficiently.
Yet the old vocabulary survives. Engineers still describe a /8 allocation as class A sized, home routers still default to class C style addressing in the 192.168 ranges, and private address blocks themselves were carved out along lines the class system drew. The private ranges, 10.0.0.0 and friends, 172.16.0.0 and friends, 192.168.0.0 and friends, are easiest to remember as one block from each of the first three of the class a b c d e networks. Learn the classes, and modern addressing reads like a dialect of a language you already speak.
Worked examples: reading class a b c d e networks from real addresses
Practice fixes the system in memory faster than any table, so run through a few addresses the way an engineer would. The address 10.45.0.8 begins with 10, inside the 0 to 127 band, so it is a class A address, and in fact part of the private block. The address 172.20.5.4 begins with 172, inside 128 to 191, so class B, also in private use in its 172.16 to 172.31 slice. The address 193.0.0.25 is class C by its first octet, a public address of the small-network kind. The address 230.1.2.3 looks ordinary but begins with 230, which places it in class D: it is a multicast group, not a machine, and no router will treat it as one. Finally, 250.0.0.1 falls in class E, reserved space you should never see assigned in production.
Working examples like these show why class a b c d e networks still earn their place in courses despite being retired: the first octet of any IPv4 address continues to tell an informed reader a story about what the address is for, and misreading it, treating a multicast group as a host or a loopback as a typo, produces very practical confusion at real terminals.
Frequently asked questions
What are class a b c d e networks in simple terms? They are the original five categories of IPv4 addresses. Classes A, B and C addressed ordinary networks of decreasing size, class D was reserved for multicast and class E for experimental use.
How can I tell the class of an IP address quickly? Look at the first number. From 0 to 127 is class A, 128 to 191 class B, 192 to 223 class C, 224 to 239 class D and 240 to 255 class E. That single glance is the everyday survival of the class a b c d e networks system.
Are classful networks still used today? Not as an allocation system; CIDR replaced it in 1993 because the fixed class sizes wasted addresses. The class names and default masks survive as shorthand and in defaults, and exams and documentation still expect you to know them.
Why does my home network use 192.168 addresses? Because 192.168.0.0 to 192.168.255.255 is a private address block drawn from the class C sized space, reserved for internal networks and never routed on the public internet. It is one of the most visible legacies of class a b c d e networks.
Conclusion
Class a b c d e networks were the internet’s first filing system: five fixed sizes, readable at a glance from the first octet, elegant and ultimately too rigid for a growing network. CIDR replaced the mechanism but kept the memory, and anyone who can recite the five ranges holds the key to a great deal of everyday networking language.
To go deeper on addressing, our guide to the class C IP address range zooms in on the most familiar class, our explainer on the CIDR IP address covers the system that replaced classes, and our look at an Axis default IP address shows addressing at the device level.
