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

What Are IP Address Classes?

Before modern CIDR (Classless Inter-Domain Routing) notation, the internet used a system called classful addressing to organize IPv4 addresses. Introduced in the early 1980s, this system divided the entire 32-bit IPv4 address space into five classes — A through E — each with a fixed allocation of network and host bits.

The class of an IP address was determined by its first few bits (or equivalently, the first octet value). This made routing simple: a router could determine how large the network portion of an address was just by looking at the first byte, without needing any additional information.

While classful addressing has been officially obsolete since 1993 (replaced by CIDR), understanding IP classes remains important because:

• Legacy systems and documentation still reference Class A/B/C
• The private IP ranges (RFC 1918) are still described in class terms
• Many networking certifications (CompTIA Network+, CCNA) test classful addressing knowledge
• Understanding classes explains why certain IP ranges behave differently

Class A: Massive Networks

Class A addresses are identified by a leading binary 0 bit, meaning the first octet value is between 1 and 126. The network portion is only the first octet; the remaining three octets identify hosts.

• Range: 1.0.0.0 to 126.255.255.255
• Default subnet mask: 255.0.0.0 (or /8)
• Networks available: 126 (2⁷ - 2)
• Hosts per network: 16,777,214 (2²⁴ - 2)

Class A space was originally assigned to very large organizations and governments. For example, General Electric owned 3.0.0.0/8, MIT owned 18.0.0.0/8, and the US Defense Information Systems Agency owned multiple Class A blocks. Today, many of these have been returned and reallocated more efficiently.

Note: 127.0.0.0/8 is reserved for loopback (explained in our loopback address guide) and is not a usable Class A range. The private 10.0.0.0/8 block is technically a Class A range reserved for private use.

Class B and Class C: Medium and Small Networks

Class B addresses begin with binary 10, putting the first octet between 128 and 191. The first two octets are the network portion; the last two identify hosts.

• Range: 128.0.0.0 to 191.255.255.255
• Default subnet mask: 255.255.0.0 (or /16)
• Networks available: 16,384 (2¹⁴)
• Hosts per network: 65,534 (2¹⁶ - 2)

Class B was assigned to universities, mid-size companies, and regional ISPs. The private range 172.16.0.0 to 172.31.255.255 spans 16 Class B-sized blocks.

Class C addresses begin with binary 110, first octet between 192 and 223. Three octets are the network; only the last octet identifies hosts.

• Range: 192.0.0.0 to 223.255.255.255
• Default subnet mask: 255.255.255.0 (or /24)
• Networks available: 2,097,152 (2²¹)
• Hosts per network: 254 (2⁸ - 2)

Class C is the most familiar — your home network is almost certainly in the Class C private range 192.168.0.0/16. Class C networks are the smallest classful allocation at 254 usable hosts, which proved wasteful as organizations needing 300 hosts had to be assigned two Class C blocks.

Class D (Multicast) and Class E (Reserved)

Class D addresses are reserved for IP multicast. Unlike unicast (one-to-one) or broadcast (one-to-all), multicast delivers data to a group of specific interested receivers simultaneously.

• Range: 224.0.0.0 to 239.255.255.255
• First octet: 224–239 (binary starts with 1110)

Multicast is used for video conferencing, streaming, and routing protocols. Common multicast addresses include 224.0.0.1 (all hosts on segment), 224.0.0.2 (all routers), and 239.0.0.0/8 (administratively scoped for private use).

Class E addresses are experimental and reserved for research — they've never been used in production deployments.

• Range: 240.0.0.0 to 255.255.255.255
• First octet: 240–255 (binary starts with 1111)
• 255.255.255.255 is specifically the limited broadcast address

Why Classful Addressing Was Replaced by CIDR

Classful addressing had a fundamental flaw: massive waste. A company needing 300 IP addresses had to receive an entire Class B block (65,534 addresses), leaving 65,234 addresses unused. Meanwhile, organizations needing 300 couldn't be adequately served by two Class C blocks. This inefficiency accelerated IPv4 exhaustion.

In 1993, RFC 1519 introduced CIDR (Classless Inter-Domain Routing), which abandoned fixed class sizes in favor of variable-length subnet masks. CIDR allows any network size — a company needing 300 IPs gets a /23 block (512 addresses), much more efficient than a Class B.

CIDR notation uses a slash followed by the number of network bits: 192.168.1.0/24 means the first 24 bits are the network portion. This is far more flexible than the old class system. Learn more in our CIDR notation guide.

Today, the internet operates entirely on CIDR. Routing tables use CIDR prefixes, ISPs allocate CIDR blocks, and the concept of "Class A/B/C" is purely historical — though still widely referenced in documentation and networking education.

Frequently Asked Questions