4xx is when the client did something wrong: (1) asked for a resource which does not exist - 404 (2) does not have permission to access the resource -

4xx is when the client did something wrong: (1) asked for a resource which does not exist - 404 (2) does not have permission to access the resource - 403 (3) is not authenticated - 401 (4) not correct / enough parameters provided - bad request 400. (5) doing a post request where a put request was supposed to be made - 405 5xx is when the server is doing something wrong. Most common is whenever something unexpected has happened - 500 Internal Server Error