IP Address Classes (A, B, C, D, E)
Understand the original classful IP addressing system with Class A through E, their ranges, and why CIDR replaced it.
The Classful Addressing System
In the early days of the internet, IPv4 addresses were divided into five classes based on the first few bits of the address. This system, defined in RFC 791, determined how many networks and hosts each class could support.
While classful addressing has been replaced by CIDR (Classless Inter-Domain Routing), understanding classes remains essential for networking exams, legacy systems, and grasping how IP allocation evolved.
The Five IP Address Classes
| Class | First Bits | Range | Default Mask | Networks | Hosts/Network |
|---|---|---|---|---|---|
| A | 0 | 1.0.0.0 -- 126.255.255.255 | /8 | 126 | 16,777,214 |
| B | 10 | 128.0.0.0 -- 191.255.255.255 | /16 | 16,384 | 65,534 |
| C | 110 | 192.0.0.0 -- 223.255.255.255 | /24 | 2,097,152 | 254 |
| D | 1110 | 224.0.0.0 -- 239.255.255.255 | N/A | Multicast | N/A |
| E | 1111 | 240.0.0.0 -- 255.255.255.255 | N/A | Reserved | N/A |
Class A was intended for very large organizations, Class B for medium-sized networks, and Class C for small networks. Classes D and E are not assigned to hosts.
Identifying the Class from an Address
You can determine the class by looking at the first octet:
- 1--126 = Class A
- 128--191 = Class B
- 192--223 = Class C
- 224--239 = Class D (multicast)
- 240--255 = Class E (experimental)
Note that 127.x.x.x is reserved for loopback and does not belong to any class. The address 0.0.0.0 is also special (default route / "this network").
Why Classful Addressing Failed
The classful system wasted enormous numbers of addresses:
- A Class A network provides over 16 million host addresses. Very few organizations need that many, yet the entire block was allocated.
- A Class C network provides only 254 hosts. Many organizations needed more but could not justify a Class B.
- There was no middle ground between 254 and 65,534 hosts.
This inefficiency accelerated IPv4 address exhaustion and led to the adoption of CIDR in 1993 (RFC 1519), which allows arbitrary prefix lengths like /20 or /27 instead of fixed class boundaries.
Classes in Modern Networking
Today, classful addressing is largely historical. However, you will still encounter class-based terminology in:
- Default subnet masks -- Tools sometimes auto-suggest /8, /16, or /24 based on the address class.
- Private address ranges -- RFC 1918 ranges align with class boundaries (10.0.0.0/8 is Class A, 172.16.0.0/12 spans Class B, 192.168.0.0/16 is Class C).
- Certification exams -- CompTIA Network+, CCNA, and similar exams still test classful knowledge.