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Here are the steps to validate an FCM registration token on a server:

1. Create a HTTP request: Make a HTTP request to Firebase server with the registration token and FCM API key. You can use any HTTP client like Postman or cURL for this.

2. Check the response status: Check the response status code to ensure that the request was successful, which is usually a 200 OK status.

3. Parse the JSON response: The response from Firebase server is in JSON format. Parse this response to extract the response data like "success" or "failure".

4. Check the response data: The response data contains information about the status of the registration token. Check if the response data contains a "success" message, which indicates that the registration token is valid.

5. Handle errors: If the response data contains a "failure" message, then the registration token is invalid. Handle errors accordingly.

6. Handle server errors: If there are internal server errors, like a 500 or 503 error, then handle the errors appropriately. Retry the request after some time, or implement a retry mechanism.

7. Store the token: Once the token is validated, store it in your database or application, so you can use it later for sending notifications to that specific mobile device.