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Browser response codes and what they mean

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When you try to load a website in your browser but something goes wrong, your browser will usually give you an error code correlating to what’s at fault.  Here are the most common errors, what they mean – and what you’ll need to do, to fix it.

4xx errors

These are “client errors”, ie the client (you) is requesting something you’re not allowed or doesn’t exist:

400 = Bad Request (eg you’ve used the wrong syntax in the URL you typed)

401 = Unauthorised (authentication is required but hasn’t yet been given)

403 = Forbidden (the request is valid, but the server is refusing to respond – usually because the client doesn’t have permission to access the resource)

404 = Not Found (the resource isn’t found – the most common error, usually because you’ve typed the URL wrong)

405 = Method Not Allowed (the request method is not allowed, eg you’ve tried to POST data to a URL that requires GET)

5xx errors

These are “server errors”, where the problem is on the server end.

500 = Internal Server Error (a generic error, where something unexpected has happened – usually in the code – and the server is unable to respond as a result)

501 = Not Implemented (the server doesn’t recognise or can’t respond to the request, but could do in the future)

502 = Bad Gateway (the server is a gateway or proxy, but got an invalid response from the upstream server; either because it’s overloaded and will be able to respond to a subsequent request; or possibly because of errors in the code of the upstream server)

503 = Service Unavailable (usually a temporary error, because of too many requests to the server or it’s down for maintenance)

504 = Gateway Timeout (the server is a gateway or proxy, but didn’t get a response from the upstream server; usually because it’s overloaded)

You can see all the possible HTTP status codes here.