IP Address Class Finder: A/B/C/D/E + Custom Mask + Batch

IP Address Class Finder

Binary bit breakdown · RFC range detection · custom mask · batch mode

Runs entirely in your browser — no address is ever sent to a server.

Accepts dotted-decimal, CIDR notation, or an IPv6 address
Any valid contiguous mask — the network/host boundary is recalculated from it
Up to 50 addresses · CIDR notation allowed

Result

Enter an IPv4 address to see its class, the binary bits that decide it, the default mask, the network/host split and the RFC range it belongs to

Classful addressing (A/B/C/D/E) is taught for exams and legacy context; modern networks use classless CIDR routing, so the "default mask" is historical rather than operational. Range flags cite the defining RFC where one exists. The IPv6 check identifies the address type from its leading bits only — it is not a full IPv6 subnetting analysis.

IP Address Class Finder: Class A to E with Binary Breakdown and RFC Ranges

Classful addressing hasn’t been how the internet actually routes traffic since 1993, but it’s still taught everywhere because it’s the simplest way to understand how an IP address’s structure works and it still shows up in every networking exam and certification. This tool identifies the class of any IPv4 address from its leading bit pattern and shows the actual per-octet binary breakdown so you can see exactly which bits made that decision, not just the final letter. It also flags private, public, loopback, APIPA, multicast, CGNAT and other special ranges with the RFC that defines each one, includes a custom subnet mask override for recalculating the network and host split with any real mask and a batch mode for checking a whole list of addresses at once.

How to Use

This tool has two tabs: Single Address and Batch Mode.

Step 1: Checking a single address

  • Type your address into the IP Address field. It accepts plain dotted-decimal like 192.168.1.1, CIDR notation like 192.168.1.1/24, or an IPv6 address like fe80::1.
  • If you want to see the network and host split using a real, non-default mask instead of the historical classful default, tick “Use custom subnet mask” and enter your mask in the Custom Subnet Mask field that appears, for example 255.255.255.192. The tool recalculates the network and host bit boundary from whatever valid mask you enter here.
  • Tap Analyse Address. For an IPv4 address, the result shows its class (A through E), the per-octet binary breakdown highlighting the leading bits that decided the class, the default classful subnet mask, the network and host bit boundary, and which special range it falls into, private, public, loopback, APIPA, multicast and so on, along with the exact RFC that defines that range. For an IPv6 address, the result shows its address type, global unicast, link-local, unique local, multicast, loopback or unspecified, based on its leading bits, with the relevant RFC.

Step 2: Checking many addresses at once

  • Switch to the Batch Mode tab.
  • Paste or type up to 50 IP addresses into the box, one per line. CIDR notation is accepted here too.
  • Tap Analyse All to get a results table showing the class, range type and RFC for every single address in your list in one pass.
  • Once you have batch results, use Download CSV to save the full table as a spreadsheet file.

Step 3: Exporting a single result

  • Use Print / PDF for a clean printable copy or Copy Result to paste the figures elsewhere.

Key Features

  • Identifies IPv4 Class A through E from the address’s actual leading bit pattern
  • Full per-octet binary breakdown, showing exactly which bits determined the class, not just the final answer
  • Flags private, public, loopback, APIPA, CGNAT, multicast, documentation and reserved ranges, each with its defining RFC
  • Custom subnet mask override that recalculates the network and host boundary for any valid, real-world mask, not just the historical classful default
  • Batch mode for checking up to 50 addresses in one pass, with a full comparison table
  • CSV export for the batch results table
  • Basic IPv6 address-type identification from its leading bits, global unicast, link-local, unique local, multicast, loopback or unspecified
  • Runs entirely in your browser. No address you type is ever sent to a server
  • Export as PDF or copy the result.

Formula / Logic Used

Class Detection from Leading Bits

Class A: first bit 0Class B: first bits 10Class C: first bits 110\text{Class A: first bit } 0 \qquad \text{Class B: first bits } 10 \qquad \text{Class C: first bits } 110
Class D: first bits 1110Class E: everything else\text{Class D: first bits } 1110 \qquad \text{Class E: everything else}

Default Classful Mask and Boundary

Each class carries a fixed default network/host split: Class A uses /8 (255.0.0.0), Class B uses /16 (255.255.0.0) and Class C uses /24 (255.255.255.0). Classes D and E don’t use a traditional subnet mask at all, since they serve special purposes rather than host addressing.

Custom Mask to Prefix Length

Prefix=position of the first 0 bit in the mask’s binary formPrefix = \text{position of the first 0 bit in the mask's binary form}

The tool verifies the mask is contiguous, all 1s followed by all 0s, with no mixed pattern, before recalculating the network and host boundary from it.

Special Range Detection

Each octet combination is checked against known reserved blocks in order, for example, 10.x.x.x and 192.168.x.x map to RFC 1918 private space, 169.254.x.x maps to APIPA under RFC 3927 and 224.x.x.x through 239.x.x.x maps to multicast under RFC 5771, among others.

Who Should Use This Tool

Networking students and anyone preparing for CompTIA Network+, CCNA or similar certifications who needs to work through classful addressing exercises quickly. also useful for IT support staff checking a batch of device addresses at once or verifying whether a specific address falls in a private, reserved or special-use range.

Frequently Asked Questions (FAQs)

1. How do I know what class an IP address belongs to?

Look at the binary pattern of the leading bits in the first octet: a leading 0 is Class A, leading 10 is Class B, leading 110 is Class C, leading 1110 is Class D and anything else is Class E. This tool shows you that exact bit pattern alongside the final class so you can see the reasoning, not just the answer.

2. Why does this tool still teach classful addressing if the internet uses CIDR now?

Classful addressing hasn’t been used for real-world routing since 1993, but it’s still the standard way networking is first taught, and it still appears throughout certification exams and legacy documentation. This tool clearly separates the historical classful default from a custom, real-world mask you can enter yourself.

3. Is 172.17.0.0 a Class B address or a private address?

Both, and this isn’t a contradiction. It falls within the Class B range because its first octet is between 128 and 191 and it also falls within the RFC 1918 private range 172.16.0.0/12, which happens to sit entirely inside the Class B space. This tool shows both the class and the private/public range status together for exactly this reason.

4. How do I check the network and host split for a mask that isn’t the classful default?

Tick “Use custom subnet mask” and enter your actual mask, for example 255.255.255.192 for a /26 network. The tool verifies the mask is a valid, contiguous pattern and recalculates the network and host boundary from it directly, rather than assuming the class’s historical default.

5. Can I check a whole list of IP addresses at once instead of one at a time?

Yes, switch to Batch Mode, paste up to 50 addresses, one per line, and tap Analyse All. You’ll get a single table showing the class, range type, and RFC reference for every address, which you can then download as a CSV file.

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