Legacy IPv4 Class Identifier
Explain historical classful address categories.
Legacy IPv4 Class Identifier: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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How Legacy IPv4 Class Identifier Works?
The Legacy IPv4 Class Identifier works by reading the leading bits of the first octet and mapping them to Class A, B, C, D, or E, as defined in RFC 791 before CIDR replaced classful addressing.
Enter an address
Type an address such as
172.20.5.9.Read the first octet
The first octet, 172, is 10101100 in binary.
Match the leading bits
Leading bits
10mark Class B.Return the class details
The tool returns the class, the default mask (
255.255.0.0), and the historical classful network (172.20.0.0).
Legacy IPv4 Class Identifier Formula
The Legacy IPv4 Class Identifier formula is Class = the first-octet range: A 0–127, B 128–191, C 192–223, D 224–239, E 240–255.
Class A
first bit = 0Default /8, 128 networks.
Class B
first bits = 10Default /16, 16,384 networks.
Class C
first bits = 110Default /24, 2,097,152 networks.
Class D and E
1110 = D, 1111 = EMulticast and reserved space, with no default mask.
Worked example: 172.20.5.9
172 = 10101100 → leading bits 10 Class = B Default mask = 255.255.0.0 (/16) Classful net = 172.20.0.0
Legacy IPv4 Class Identifier Conversion Table
The Legacy IPv4 Class Identifier conversion table summarizes the 5 historical IPv4 classes: leading bits, first-octet range, default mask, network count, and hosts per network.
| Class | Leading Bits | First Octet | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|---|
| A | 0 | 0 – 127 | 255.0.0.0 (/8) | 128 | 16,777,214 |
| B | 10 | 128 – 191 | 255.255.0.0 (/16) | 16,384 | 65,534 |
| C | 110 | 192 – 223 | 255.255.255.0 (/24) | 2,097,152 | 254 |
| D | 1110 | 224 – 239 | None (multicast) | — | — |
| E | 1111 | 240 – 255 | None (reserved) | — | — |
Sample results: 10.4.4.4 → Class A, 150.1.1.1 → Class B, 200.1.1.1 → Class C, 230.1.1.1 → Class D, 250.1.1.1 → Class E.
What is Legacy IPv4 Class Identifier?
The Legacy IPv4 Class Identifier is a tool that shows which historical address class, A through E, an IPv4 address belonged to under classful addressing.
Classful addressing, defined in RFC 791 in 1981, tied the network size to the first octet. Class A networks held 16.7 million hosts and Class C networks held 254. The fixed sizes wasted large amounts of space, and RFC 1519 introduced CIDR in 1993 to replace them.
Modern routing ignores classes. The identifier exists for historical context, older documentation, legacy device defaults, and certification exams that still test classful concepts.
What are the uses of Legacy IPv4 Class Identifier?
There are 4 main uses of the Legacy IPv4 Class Identifier, including exam preparation, legacy device support, and documentation reading.
Preparing for certification exams
CCNA and Network+ exams still ask for the class of an address and its default mask.
Supporting legacy devices
Older equipment and RIPv1 assume classful boundaries. The class explains automatic summarization.
Reading old documentation
Network diagrams from the 1990s refer to "a Class B". The tool maps those terms to modern prefixes.
Teaching the history of CIDR
Instructors show how fixed classes wasted space and why CIDR replaced them.