# `ASN_INDEX_UNAVAILABLE` (HTTP 501): the ASN prefix index has not been built on this server

GET /api/v1/ip/asn/{asn} needs the index `lookup:index-asns` builds from the ASN database, and it is not installed. Nothing is charged.

The ASN database answers "which system does this address belong to"; the ASN endpoint answers the reverse, "which prefixes does this system hold", and that needs the database turned inside out once a month after it syncs. Until that has run there is nothing to answer from, and saying so is more honest than an empty prefix list.

Every address lookup is unaffected: asn and org on an address come straight from the database.

**What to do.** Retry later. If it persists, the operator has not run lookup:index-asns since the last ASN database sync.

```json
{
    "success": false,
    "error": {
        "code": "ASN_INDEX_UNAVAILABLE",
        "message": "The ASN prefix index has not been built on this server yet.",
        "request_id": "req_01m1kgdm4xngzmbmff68g94w0c"
    }
}
```

Reference: https://spaw.co/docs/errors/ASN_INDEX_UNAVAILABLE