Labelling this as free is a scam since you need a) a registration on their site and b) only have a small quota for free before you have to pay up.
I agree with droomagentur, for a free account, you are tied with 500 Queries. Please see their pricing here: https://whoisapi.whoisxmlapi.com/pricing
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I agree with droomagentur, for a free account, you are tied with 500 Queries. Please see their pricing here: https://whoisapi.whoisxmlapi.com/pricing