# IP address blocking

Each project can refuse Challenge traffic from addresses you choose. Visitors from those addresses are never issued a challenge, so their submissions never reach you. Classify currently does not use this list.

## Blocking an address

Open the project in the dashboard and go to its Settings tab. Under `Blocked IP addresses`, add one address or CIDR range per line, then press `Save changes`. IPv4 and IPv6 both work. A single address blocks only that address.

Shell

```bash
# A single address
203.0.113.4

# An IPv4 range, in CIDR notation
198.51.100.0/24

# IPv6 works the same way
2001:db8::/32
```

## What a blocked visitor sees

The challenge request is refused, so `challenge()` rejects and nothing is sent. Blocked requests never count toward your allowance.

## Choosing what to block

Prefer single addresses to ranges. Mobile carriers, universities, offices and VPNs put many people behind few addresses. Blocking one of those ranges turns away everyone behind it, and the source you meant to stop moves to another address in minutes.

Block a specific, persistent source you can already identify. Leave everyday spam to the challenge, which costs a real visitor a moment, not their access.