Special-Purpose Address Classifier
Use a selected address registry snapshot.
Special-Purpose Address Classifier: 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 Special-Purpose Address Classifier Works?
The Special-Purpose Address Classifier works by comparing an IPv4 address against every block in a selected snapshot of the IANA IPv4 Special-Purpose Address Registry and returning the most specific match.
Enter an address or prefix
Type a value such as
100.72.10.4or198.18.0.0/15.Select the registry snapshot
The classifier uses a dated copy of the IANA registry defined by RFC 6890, so every result is reproducible.
Find the matching block
The classifier tests the address against each registry block and keeps the longest match.
Return the attributes
The result shows the block name, the defining RFC, and whether the block is globally reachable.
Special-Purpose Address Classifier Formula
The Special-Purpose Address Classifier formula is Match = the registry block with the largest prefix p where (IP AND Mask(p)) = Block Network.
Membership test
(IP AND Mask(p)) = NRuns for every registry block.
Best match
max(p) over all matchesThe most specific block wins.
No match
Result = Global unicastAddresses outside every block are ordinary public addresses.
Worked example: 100.72.10.4
100.64.0.0/10 → mask 255.192.0.0 72 AND 192 = 64 → 100.64.0.0 = block network ✓ Name = Shared Address Space (CGNAT) RFC = RFC 6598 Global = No
Special-Purpose Address Classifier Conversion Table
The Special-Purpose Address Classifier conversion table lists 16 special-purpose IPv4 blocks with the block size, the defining RFC, and global reachability.
| Block | Name | Addresses | RFC | Globally Reachable |
|---|---|---|---|---|
| 0.0.0.0/8 | "This network" | 16,777,216 | RFC 791 | No |
| 10.0.0.0/8 | Private-Use | 16,777,216 | RFC 1918 | No |
| 100.64.0.0/10 | Shared Address Space | 4,194,304 | RFC 6598 | No |
| 127.0.0.0/8 | Loopback | 16,777,216 | RFC 1122 | No |
| 169.254.0.0/16 | Link-Local | 65,536 | RFC 3927 | No |
| 172.16.0.0/12 | Private-Use | 1,048,576 | RFC 1918 | No |
| 192.0.0.0/24 | IETF Protocol Assignments | 256 | RFC 6890 | No |
| 192.0.2.0/24 | Documentation (TEST-NET-1) | 256 | RFC 5737 | No |
| 192.88.99.0/24 | 6to4 Relay Anycast (deprecated) | 256 | RFC 7526 | No |
| 192.168.0.0/16 | Private-Use | 65,536 | RFC 1918 | No |
| 198.18.0.0/15 | Benchmarking | 131,072 | RFC 2544 | No |
| 198.51.100.0/24 | Documentation (TEST-NET-2) | 256 | RFC 5737 | No |
| 203.0.113.0/24 | Documentation (TEST-NET-3) | 256 | RFC 5737 | No |
| 224.0.0.0/4 | Multicast | 268,435,456 | RFC 5771 | Scoped |
| 240.0.0.0/4 | Reserved | 268,435,456 | RFC 1112 | No |
| 255.255.255.255/32 | Limited Broadcast | 1 | RFC 919 | No |
What is Special-Purpose Address Classifier?
The Special-Purpose Address Classifier is a tool that identifies whether an IPv4 address belongs to a reserved, private, documentation, or other special-purpose block, and which RFC defines that block.
The Internet Assigned Numbers Authority (IANA) maintains the IPv4 Special-Purpose Address Registry. Each entry lists a block, a name, an RFC, and flags such as "globally reachable" and "forwardable". The classifier works from a dated snapshot of that registry, which keeps results consistent across runs.
About 592 million IPv4 addresses, roughly 13.8 % of the 4.29 billion total, fall into special-purpose or reserved space. Most of the share comes from the multicast block (268 million) and the reserved 240.0.0.0/4 block (268 million).
What are the uses of Special-Purpose Address Classifier?
There are 5 main uses of the Special-Purpose Address Classifier, including bogon filtering, log triage, and test data selection.
Building bogon filters
Edge routers drop packets from special-purpose sources. The classifier confirms which blocks belong in the filter.
Triaging security logs
An alert from
100.72.10.4points to a carrier-grade NAT customer, not to a public host.Choosing documentation addresses
RFC 5737 blocks are safe for examples and training material. The classifier confirms the right block.
Validating customer input
Signup forms reject loopback and link-local addresses entered as public server IPs.
Explaining unreachable addresses
Support teams show users that
169.254.x.xmeans DHCP failed, not that the server is down.