You're allowed up to 600 queries per hour by default. Once this limit is reached, all of your requests will result in HTTP 444, forbidden, until your quota is cleared.
The HTTP API takes GET requests in the following schema:
https://ip.222.at/{format}/{IP_or_hostname}
Supported formats are:
csv,
xml,
json,
jsonp
and
iponly.
If no IP or hostname is provided or "iponly" is called, then your own IP address is looked up.
If you call the "iponly" format your current own IPv4 or IPv6 is shown in plain text.
JSON: https://ip.222.at/json/
JSON (opt. w/ Callback): https://ip.222.at/json/8.8.8.8?callback=test
IPONLY: https://ip.222.at/iponly/