HTTP Status Code

An HTTP status code is a three-digit number the server sends with every response to say whether the request succeeded, was redirected, or hit an error.

The status code is the shared language of browser and server, grouped into five families: 2xx success, 3xx redirect, 4xx a request-side error (such as 404 not found or 401 unauthorized), and 5xx a server-side error. For analyzing a site's health, separating these families is vital because they mean different things: a 500 is an alarm, a 404 may be a broken link or normal behavior, and a 301 is a deliberate redirect. But there is a key measurement limit: the status code is known to the server and recorded in the server log; an analytics script running inside the browser on a page usually does not have the main navigation's status code. So status-code analysis is naturally the server log's job, not a browser script's.