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Using Sticky Sessions:

KeyValueDurationExamples
sessionAny unique identifier of up to 255 charactersNo expiration0, abc, session123
Add a session identifier to keep your requests on the same IP address: 
curl --proxy http://isp.joinmassive.com:8000 -U '{PROXY_USERNAME}-session-37:{API_KEY}' ip-api.com
When using sticky sessions:
  1. The first request with a session ID linked to a specific IP address.
  2. All subsequent requests with the same session ID will use that same IP address.
  3. Sessions continue indefinitely unless the session identifier is changed.
  4. Requests that do not have a session ID will automatically be assigned a new IP address.