4xx family (client error)

These are the standard error status codes which the client needs to interpret and handle further actions. These have nothing to do with the server. A wrong request format or ill-formed REST method can cause these errors. Of these, the most frequent status codes API developers use are 400, 401, 403, 404, and 405:

• 400 (Bad Request) is returned when the server cannot understand the client request.
• 401 (Unauthorized) is returned when the client is not sending the authorization information in the header.
• 403 (Forbidden) is returned when the client has no access to a certain type of resources.
• 404 (Not Found) is returned when the client request is on a resource that is nonexisting.
• 405 (Method Not Allowed) is returned if the server bans a few methods on resources. GET and HEAD are exceptions.