IP Class Checker |
IPv4 class checking identifies which of the five classful address categories — A, B, C, D, or E — an IPv4 address belongs to, based on the value of its first octet. The classification system was defined in RFC 791 (1981) and determined the default network boundaries and subnet masks before Classless Inter-Domain Routing (CIDR) replaced it in 1993 (RFC 4632). Classes A, B, and C are unicast and carry default masks of /8, /16, and /24 respectively; Class D is reserved for multicast; Class E is reserved for experimental use.
The table below shows the five IPv4 address classes, their first octet ranges, default subnet masks, CIDR prefixes, and maximum unicast host counts, as defined in RFC 791. Class D and Class E have no subnet mask — they are not used for host addressing.
| Class | First Octet Range | IP Range | Default Mask | CIDR | Max Hosts / Network | Purpose |
|---|---|---|---|---|---|---|
| A | 1 – 126 | 1.0.0.0 – 126.255.255.255 | 255.0.0.0 | /8 | 16,777,214 | Large unicast networks |
| A (Loopback) | 127 | 127.0.0.0 – 127.255.255.255 | 255.0.0.0 | /8 | — | Loopback only — not routed (RFC 1122) |
| B | 128 – 191 | 128.0.0.0 – 191.255.255.255 | 255.255.0.0 | /16 | 65,534 | Medium unicast networks |
| C | 192 – 223 | 192.0.0.0 – 223.255.255.255 | 255.255.255.0 | /24 | 254 | Small unicast networks |
| D | 224 – 239 | 224.0.0.0 – 239.255.255.255 | N/A | N/A | N/A | Multicast groups (RFC 1112) |
| E | 240 – 255 | 240.0.0.0 – 255.255.255.255 | N/A | N/A | N/A | Reserved / Experimental (RFC 1112) |
Each IPv4 address class is encoded in the leading bits of its first octet. A router or host can determine the class in a single operation by inspecting those bits — no lookup table is required. This binary identification method is the basis of the classful routing logic defined in RFC 791.
| Class | Leading Bits | Binary Pattern (First Octet) | First Octet Decimal Range |
|---|---|---|---|
| A | 0 | 0xxxxxxx | 1 – 126 |
| B | 10 | 10xxxxxx | 128 – 191 |
| C | 110 | 110xxxxx | 192 – 223 |
| D | 1110 | 1110xxxx | 224 – 239 |
| E | 1111 | 1111xxxx | 240 – 255 |
Enter any IPv4 address to classify it instantly per RFC 791 classful addressing rules.
IPv4 classful addressing, defined in RFC 791 (September 1981), divides the 32-bit address space into five classes by inspecting the leading bits of the first octet. Class A (leading bit 0) allocates 8 bits to the network identifier and 24 bits to host addressing, yielding up to 16,777,214 usable hosts per network. Class B (leading bits 10) allocates 16 bits to each, supporting up to 65,534 hosts. Class C (leading bits 110) allocates 24 network bits and 8 host bits, supporting 254 hosts per network.
The gap at first octet 127 is intentional — RFC 1122 (Section 3.2.1.3) reserves the entire 127.0.0.0/8 block for loopback. Packets addressed to any 127.x.x.x address are processed locally and never forwarded. RFC 1918 reserves three blocks within Classes A, B, and C for private use: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B range), and 192.168.0.0/16 (Class C). CIDR (RFC 4632, 1993) replaced classful routing with variable-length prefix lengths, but the class system remains foundational to networking certification exams (CCNA, CompTIA Network+) and is directly referenced in legacy network documentation and routing configurations.
Legacy network documentation. A network administrator reviewing a legacy routing table from the early 1990s encounters entries in the 172.20.x.x range and needs to confirm the classful boundary. The IP Class Checker identifies these as Class B addresses (default mask /16, up to 65,534 hosts), confirming they fall within RFC 1918 private space and that the default boundary starts at 172.0.0.0 — not 172.16.0.0, which is a CIDR refinement.
Certification exam study. A student preparing for the CompTIA Network+ exam enters a series of test addresses — 10.0.0.1 (Class A), 172.16.0.1 (Class B), 192.168.1.1 (Class C), 224.0.0.5 (Class D), and 240.0.0.1 (Class E) — to verify the expected class, default mask, and binary leading-bit pattern against the exam material. Bulk mode classifies all five in a single operation, with the subnet mask column confirming the default for each.
Firewall log analysis. A security analyst reviewing firewall logs encounters inbound traffic sourced from 243.0.0.x. The IP Class Checker identifies this as Class E (Reserved/Experimental) — addresses in this range are not publicly assigned to any organization and should never appear as legitimate source addresses on a production network. The result flags the traffic for investigation.