A 4xx code indicates an error caused by the user, whereas 5xx codes tell the client that they did everything correctly and it’s the server itself who caused the problem. 401 (unauthorized), 403 (forbidden), 404 (not found), and so on all mean the client has to fix the problem on their side before trying again.400s: Client error codes indicating that there was a problem with the request. 500s: Server error codes indicating that the request was accepted, but that an error on the server prevented the fulfillment of the request. Within each of these classes, a variety of server codes exist and may be returned by the server.