Understanding HTTP Status Codes

HTTP status codes are numerical responses sent by a web server when a client, such as a browser or monitoring tool, makes a request. Understanding these codes is essential for diagnosing website issues and ensuring optimal uptime.

Why Status Codes Matter

Status codes help you determine:

Whether a website is online or offline
If a page has moved or changed
Server or client errors
Possible security restrictions

For example, a 200 OK means the website is accessible, whereas a 503 Service Unavailable suggests server problems.

Categories of HTTP Status Codes

HTTP status codes are grouped into five main categories:

1xx: Informational

100 Continue – The server has received the request headers and the client should proceed.
101 Switching Protocols – Server agrees to switch protocols (e.g., HTTP → WebSocket).

These codes are rarely seen by end-users but are important in debugging complex applications.

2xx: Success

200 OK – The request succeeded, and the server returned the requested content.
201 Created – A new resource was successfully created (common in APIs).
204 No Content – The request succeeded, but there is no content to return.

These codes indicate that the website or API is responding correctly.

3xx: Redirection

301 Moved Permanently – Resource has permanently moved to a new URL.
302 Found – Resource temporarily available at a different URL.
304 Not Modified – Content has not changed; the browser can use cached data.

Redirection codes are crucial for SEO and proper routing.

4xx: Client Errors

400 Bad Request – The request could not be understood due to invalid syntax.
401 Unauthorized – Authentication is required or failed.
403 Forbidden – Server understood the request but refuses to authorize it.
404 Not Found – Resource not found at the requested URL.
429 Too Many Requests – Rate limiting; client is sending too many requests.

These errors often indicate problems with the request itself or the user's access permissions.

5xx: Server Errors

500 Internal Server Error – Unexpected condition preventing fulfillment of the request.
502 Bad Gateway – Invalid response from an upstream server.
503 Service Unavailable – Server temporarily overloaded or under maintenance.
504 Gateway Timeout – Server did not receive a timely response from upstream.

Server errors are critical for monitoring, as they signal backend problems that affect all users.

Checking Status Codes Programmatically

You can use tools like curl or monitoring scripts to check HTTP status codes:

curl -I https://example.com

Example output:

HTTP/2 200
date: Wed, 24 Sep 2025 10:00:00 GMT
content-type: text/html; charset=UTF-8

A 200 status confirms the site is up and reachable.

Real-World Applications

Website Monitoring – Detect downtime or errors automatically.
API Debugging – Identify issues with requests and responses.
SEO Optimization – Handle redirects and errors to improve search rankings.
Security Analysis – Spot unusual 4xx or 5xx patterns indicating attacks.

Conclusion

HTTP status codes are the foundation of web communication. By understanding them, developers and website owners can quickly diagnose issues, optimize performance, and ensure their sites remain accessible to users.

Pro Tip: Combining status code checks with DNS and ping tests gives a complete picture of whether a website is truly down or just experiencing a temporary service or network issue.