Identify the class and scope of an IPv4 address
Enter any IPv4 address to find its classful range (A, B, C, D or E), the default mask for that class, its typical use, and its scope — public, private, loopback or link-local. This is handy for students learning networking, for engineers quickly confirming whether an address is routable, and for anyone debugging why a connection attempt is going nowhere.
How it works
The class is read from the first octet of the dotted-decimal address. The tool then applies the matching default mask and checks the address against the standard reserved ranges (RFC 1918 private, 127.0.0.0/8 loopback, and 169.254.0.0/16 link-local) to determine scope.
| Class | First octet | Default mask | Typical use |
|---|---|---|---|
| A | 0–127 | /8 (255.0.0.0) | Very large networks |
| B | 128–191 | /16 (255.255.0.0) | Medium networks |
| C | 192–223 | /24 (255.255.255.0) | Small networks |
| D | 224–239 | none | Multicast groups |
| E | 240–255 | none | Reserved/experimental |
Worked examples
192.168.0.1 — first octet 192 falls in 192–223, so this is Class C with a default /24 mask. It also falls inside the 192.168.0.0/16 RFC 1918 block, so scope is private — not directly routable on the public internet.
10.0.5.200 — first octet 10 is in 0–127, so this is Class A with a /8 default mask. It falls inside 10.0.0.0/8 (RFC 1918), so scope is also private.
8.8.8.8 — first octet 8 is Class A, but it does not match any private, loopback, or link-local range, so scope is public and routable.
127.0.0.1 — first octet 127 is technically in the Class A range but the entire 127.0.0.0/8 block is designated as loopback. This tool flags it as loopback scope rather than merely Class A.
169.254.10.5 — Class B range, but 169.254.0.0/16 is the APIPA / link-local range assigned automatically when a device fails to get a DHCP address. Scope is flagged as link-local.
224.0.0.1 — first octet 224 is Class D: multicast only, no host addressing, no default mask.
Why classful addressing still matters in 2025
Classful addressing was largely retired when CIDR (Classless Inter-Domain Routing) was introduced in the 1990s — modern routing uses prefix lengths like /22 or /27, not fixed class boundaries. However, classful concepts still appear in:
- Networking education and certifications. CCNA, CompTIA Network+, and university networking courses still teach classes A–E because they explain why the internet’s early address space was structured the way it was.
- Default subnet mask assumptions. Some legacy devices and configurations assume default masks based on the first octet when no explicit mask is specified.
- Quick routable vs. private checks. Engineers still use “is this a 10.x or 192.168.x address?” as a fast mental filter, which maps directly to the private ranges within Classes A and C.
- Multicast configuration. Class D and its 224–239 range are still actively used for multicast routing protocols and streaming applications.
Notes
All analysis runs in your browser; no address is uploaded. For modern subnet calculations, pair this tool with a CIDR subnet calculator that works with arbitrary prefix lengths rather than the fixed class boundaries shown here.