IPv4 Address Classes Explained (A, B, C, D, E)
What the original classful addressing system was, how Class A/B/C/D/E networks differ, and why CIDR replaced it.
The five IPv4 address classes
| Class | Leading Bits | Range | Prefix | Networks | Hosts/Network | Purpose |
|---|---|---|---|---|---|---|
| A | 0xxxxxxx | 1.0.0.0 – 126.255.255.255 | /8 | 128 | 16,777,214 | Very large orgs, ISPs |
| B | 10xxxxxx | 128.0.0.0 – 191.255.255.255 | /16 | 16,384 | 65,534 | Large enterprises |
| C | 110xxxxx | 192.0.0.0 – 223.255.255.255 | /24 | 2,097,152 | 254 | Small networks |
| D | 1110xxxx | 224.0.0.0 – 239.255.255.255 | — | — | — | Multicast |
| E | 1111xxxx | 240.0.0.0 – 255.255.255.255 | — | — | — | Reserved (experimental) |
Why classes are obsolete
Classful addressing was wasteful. A company needing 300 hosts had to request a Class B (/16, 65,534 hosts), wasting over 65,000 addresses. In 1993, CIDR replaced classful addressing, allowing networks to be sized exactly as needed using any prefix length from /0 to /32.
Today, the concept of IP classes survives only in the RFC 1918 private range naming
("Class A private: 10/8", "Class B private: 172.16/12", "Class C private: 192.168/16")
and in the ip_class field returned by routers and tools for diagnostic purposes.
Frequently Asked Questions
What are the IPv4 address classes?
IPv4 originally had five classes. Class A (/8, ~16.7M hosts) for very large networks; Class B (/16, 65,534 hosts) for large enterprises; Class C (/24, 254 hosts) for small networks; Class D (224–239.x.x.x) for multicast; Class E (240–255.x.x.x) reserved. Classful addressing was replaced by CIDR in 1993.
Are IP address classes still used today?
Not for routing — CIDR replaced classful addressing in 1993. But the class names survive in RFC 1918 private range labels ("Class A private: 10/8", "Class C private: 192.168/16") and in some legacy tools. Modern networking uses CIDR prefix lengths exclusively.
What is a Class A IP address?
A Class A address has a first octet of 1–126, uses a /8 prefix, and supports up to 16,777,214 hosts per network. The private Class A range is 10.0.0.0/8 (RFC 1918). Note: 127.x.x.x is technically Class A but is entirely reserved for loopback and not usable as a network address.
Note on 127.x.x.x
127.0.0.0/8 falls in the Class A range but is reserved for loopback. It is not assigned as a Class A network.
127.0.0.1 (localhost) is the canonical loopback address.