HTTP Status Codes Explained: What 404, 500, and 503 Actually Mean

Every response a web server sends has a three-digit status code. The first digit tells you the class of response; the rest give the specific reason. Here's the short version.

2xx — Success

• 200 OK — Everything worked.
• 204 No Content — Worked, but there's nothing to return (common for background APIs).

3xx — Redirection

• 301 Moved Permanently — The URL has permanently moved. Bookmarks should update.
• 302 Found — Temporary redirect.
• 304 Not Modified — Your cached copy is still fresh; the browser reuses it.

4xx — Client errors (usually your side)

• 400 Bad Request — The server couldn't understand what you sent.
• 401 Unauthorized — You need to log in.
• 403 Forbidden — You're logged in but not allowed here.
• 404 Not Found — The URL doesn't exist on this server. Very common; usually a typo or a deleted page.
• 429 Too Many Requests — Rate-limited. Slow down.

5xx — Server errors (their side)

• 500 Internal Server Error — Something crashed inside the app. The user can't fix this.
• 502 Bad Gateway — A proxy (like Cloudflare or nginx) couldn't reach the origin.
• 503 Service Unavailable — The server is overloaded or in maintenance.
• 504 Gateway Timeout — The proxy waited too long for the origin to respond.

What our probes look for

When Down For All Or Just Me checks a site, we mark anything in the 2xx or 3xx range as "up", and anything 5xx or a network-level failure as "down". 4xx codes are treated as up because they mean the server is responding — the URL is just wrong or gated. This mirrors how Google, Pingdom, and most monitoring tools classify checks.