The earliest models of personal computers had 64k RAM or less. Programmers needed to keep things simple. They developed a classification system for program functions. Input errors got assigned to class 400.

• 400 Bad Request. The input is in the wrong syntax.

• 401 Unauthorized. The user cannot access without a username and password.

• 403 Forbidden. The user doesn’t have permission to access the file.

• 404 Not Found. You entered or linked to a URL that doesn’t exist.