ASP.NET Tutorial: Recognizing the Important HTTP Status Codes: 204, 401, 404, and 400

HTTP status codes are essential signals delivered from a server to a client (such a browser or mobile app) to indicate the result of a request in the context of web development and API integration. Status codes facilitate effective communication between developers and systems, regardless of whether a request was successful, unsuccessful due to an error, or unauthorized.

Four frequently used HTTP status codes—400, 401, 404, and 204—are the subject of this article.

401 Unauthorized: Authentication Failure

What does it mean?

The request was not completed because it lacked valid authentication credentials. The client must log in or provide a valid token to access the requested resource.

When is it used?

• Accessing a user account page without logging in
• Calling a protected API endpoint without a valid access token

Example Response

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="Access to the API"

404 Not Found: Resource Missing

What does it mean?

The server could not find the requested resource. This might be because the URL is incorrect, the resource has been deleted, or it never existed.

When is it used?

• Typing the wrong URL in the browser
• Requesting a deleted file or endpoint

Example Response

HTTP/1.1 404 Not Found
Content-Type: application/json

{
  "error": "The requested resource was not found on this server."
}

400 Bad Request: Validation Error

What does it mean?

The server could not understand the request due to invalid syntax, missing parameters, or malformed data.

When is it used?

• Submitting a form with invalid inputs
• Sending a malformed JSON payload to an API

Example Response

HTTP/1.1 400 Bad Request
Content-Type: application/json

{
  "error": "Invalid request. 'email' field is required."
}

204 No Content: Successful Deletion
What does it mean?

The request was successfully processed, but there’s no content to return. This is common when a resource is deleted or updated without returning data.

When is it used?

• Successfully deleting a user, file, or post
• Completing an update operation that doesn’t require a response body

Example Response

HTTP/1.1 204 No Content

Conclusion

HTTP status codes are more than just numbers; they provide critical context about what’s happening behind the scenes in web applications. Knowing how to interpret these codes helps developers debug faster, improve user experience, and build robust APIs.

Quick Recap

• 400 Bad Request: You sent something the server couldn’t understand
• 401 Unauthorized: You need to log in or authenticate
• 404 Not Found: The thing you’re looking for isn’t here
• 204 No Content: Success! But there’s nothing more to show

Understanding these can make troubleshooting and development smoother and more efficient.

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