IP Address Classes: A, B, C, D and E Explained

Lesson Contents

IP address classes divide the 32-bit Internet Protocol version 4 (IPv4) address space into five categories, A through E, based on the first bits of the first octet. This classful scheme fixes the boundary between the network and host portions of an address, and ties each class to a default subnet mask. That’s different from the flexible boundaries you get with modern Classless Inter-Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) designs.

In this lesson, I’ll show you how to identify each class by its first octet range and first bits. You’ll also see the default mask and network/host split for Classes A, B, and C, along with the special roles of Class D (multicast) and Class E (reserved/experimental) addresses.

Key Takeaways

Classful addressing determines an IP address’s class from the first bits of its first octet:
- Class A begins with 0.
- Class B with 10.
- Class C with 110.
- Class D with 1110.
- Class E with 1111.
The first octet’s decimal range gives you the same answer without converting to binary.
An address’s class comes from its first bits alone, and it never changes, even if you apply a subnet mask or CIDR that differs from the default.
Class A (1-126) uses mask 255.0.0.0 (/8):
- 1 octet for network, 3 octets for host.
- 126 networks, each with up to 16,777,214 usable hosts.
- 127.0.0.0/8 is reserved for loopback and isn’t usable as a network address.
Class B (128-191) uses mask 255.255.0.0 (/16):
- 2 octets for network, 2 octets for host.
- 16,384 networks, each with up to 65,534 usable hosts.
Class C (192-223) uses mask 255.255.255.0 (/24):
- 3 octets for network, 1 octet for host.
- 2,097,152 networks, each with 254 usable hosts.
The RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) aren’t a separate classification system. They’re later-defined blocks within the Class A, B, and C ranges.
Class D (224-239) has no network/host split or default mask. It’s reserved for multicast, where a single address represents a group of hosts instead of one device.
Class E (240.0.0.0-254.255.255.255) is reserved for experimental and research purposes:
- It’s not assigned for public or standard host addressing.
- 255.255.255.255 is reserved separately as the limited broadcast address.
You need to know the class ranges and default masks to understand legacy addressing rules, even though modern networks rely on classless (CIDR) addressing.

Prerequisites

To follow this lesson, you need to understand binary and decimal number conversion. The first bits of the first octet are what determine an IP address’s class. You should also be comfortable with the structure of an IPv4 address as four octets, including how a subnet mask like 255.255.255.0 separates the network and host portions of an address.

Components

This section covers the rule behind classful addressing and each of the five classes. You’ll see how first bits, address ranges, and default masks define Classes A through E. Let’s have a look.

Classful Addressing Structure

The first bits of the first octet determine which class an address belongs to, and therefore how its 32 bits split into network and host portions:

Class A starts with 0.
Class B starts with 10.
Class C starts with 110.
Class D starts with 1110.
Class E starts with 1111.

Each class has a fixed network/host boundary, which is what the default subnet mask is for. Here’s a summary of the first bits, first octet range, default mask, and network/host split for each class:

Class	First Bits	First Octet Range	Default Mask	Network/Host Split
A	`0`	1-126	`255.0.0.0` (/8)	1 octet network / 3 octets host
B	`10`	128-191	`255.255.0.0` (/16)	2 octets network / 2 octets host
C	`110`	192-223	`255.255.255.0` (/24)	3 octets network / 1 octet host
D	`1110`	224-239	N/A (multicast)	N/A
E	`1111`	240-255	N/A (reserved)	N/A

Subnetting an address doesn’t change its class. An address like 30.10.10.1 is Class A by its first bits no matter what mask you put on it, even a /24. The class and the subnet mask are two independent pieces of information.

Class A Addresses

Class A reserves the largest host space per network. It’s meant for large organizations:

First octet range: 1-126 (0 and 127 reserved)
Default subnet mask: 255.0.0.0 (/8)
1 octet for network, 3 octets for host
126 networks of 16,777,214 usable hosts each
127.0.0.0/8 matches the Class A bit pattern (first bit 0), but it’s reserved for loopback and excluded from normal host and network assignment. That’s why usable Class A addresses are listed as 1-126, not 1-127.

Class B Addresses

Class B balances network and host counts. It’s aimed at medium-to-large networks:

First octet range: 128-191
Default subnet mask: 255.255.0.0 (/16)
2 octets for network, 2 octets for host
16,384 networks of 65,534 usable hosts each

Class C Addresses

Class C provides many small networks, each with a limited number of hosts. It’s suited to small organizations:

First octet range: 192-223
Default subnet mask: 255.255.255.0 (/24)
3 octets for network, 1 octet for host
2,097,152 networks of 254 usable hosts each

The private address ranges from RFC 1918 aren’t a separate class system. They’re specific blocks carved out of the classful ranges you already know: 10.0.0.0/8 sits inside Class A, 172.16.0.0/12 sits inside Class B, and 192.168.0.0/16 sits inside Class C. A private address still belongs to its original class. Internet providers drop packets with private source addresses.

Class D Addresses

Class D isn’t used for standard host addressing. It’s reserved for multicast group communication:

First octet range: 224-239
No network/host split and no default subnet mask
A Class D address identifies a group of hosts, and the network delivers packets to every member of that group.

Class E Addresses

Class E is reserved for experimental and research purposes. It’s not used for general host addresses:

First octet range: 240-255
Usable experimental range: 240.0.0.0-254.255.255.255
255.255.255.255 is the limited broadcast address, not an experimental one.
Not allocated for public or standard network host assignment.

RFC 1112 and RFC 1812 define Class E as the first four bits 1111, which spans 240.0.0.0 to 255.255.255.255. In practice, 255.255.255.255 is used as the broadcast address, so Class E’s usable experimental range is 240.0.0.0-254.255.255.255.

Conclusion

You have now learned how classful addressing divides the 32-bit IPv4 address space into Classes A through E. The first bits of the first octet determine the class, the network/host boundary, and the default subnet mask.

We looked at the first octet ranges and default masks for each class, from Class A’s 255.0.0.0 mask supporting huge host counts to Class C’s 255.255.255.0 mask supporting many small networks. We also saw that Class D and Class E are reserved for multicast and experimental use rather than standard host addressing.

CIDR replaced classful addressing in 1993, so you won’t configure a router with classful rules today. You still need this material for certification exams, though, and for understanding legacy protocols like Routing Information Protocol version 1 (RIPv1) and Interior Gateway Routing Protocol (IGRP), which auto-summarize routes at classful network boundaries whether you want them to or not. I hope you have enjoyed this lesson. If you have any questions, please leave a comment!