HTTP status codes (the 4xx and 5xx varieties) appear when there is some kind of error loading a web page. HTTP status codes are standard types of errors so you could see them in any browser, like Internet Explorer, Firefox, Chrome, etc.
Common 4xx and 5xx HTTP status codes are listed below with helpful tips to help you get past them and on to the web page you were looking for.
Note: HTTP status codes that begin with 1, 2, and 3 also exist but are not errors and aren't usually seen. If you're interested, you can see them all listed here.
The 400 Bad Request HTTP status code means that the request you sent to the website server (for example, a request to load a web page) was somehow malformed.
Since the server couldn't understand the request, it couldn't process it and instead gave you the 400 error.
The 401 Unauthorized HTTP status code means that the page you were trying to access can not be loaded until you first log on with a valid username and password.
If you've just logged on and received the 401 error, it means that the credentials you entered were invalid. Invalid credentials could mean that you don't have an account with the web site, your username was entered incorrectly, or your password was incorrect.
The 403 Forbidden HTTP status code means that accessing the page or resource you were trying to reach is absolutely forbidden.
In other words, a 403 error means that you don't have access to whatever you're trying to view.
The 404 Not Found HTTP status code means that the page you were trying to reach could not be found on the web site's server. This is the most popular HTTP status code that you will probably see.
The 404 error will often appear as The page cannot be found.
The 408 Request Timeout HTTP status code indicates that the request you sent to the website server (like a request to load a web page) timed out.
In other words, a 408 error means that connecting to the web site took longer than the website's server was prepared to wait.
500 Internal Server Error is a very general HTTP status code meaning something went wrong on the web site's server but the server could not be more specific on what the exact problem was.
The 500 Internal Server Error message is the most common "server-side" error you'll see.
The 502 Bad Gateway HTTP status code means that one server received an invalid response from another server that it was accessing while attempting to load the web page or fill another request by the browser.
In other words, the 502 error is an issue between two different servers on the Internet that aren't communicating properly.
The 503 Service Unavailable HTTP status code means the web site's server is simply not available at the moment.
503 errors are usually due to a temporary overloading or maintenance of the server.
The 504 Gateway Timeout HTTP status code means that one server did not receive a timely response from another server that it was accessing while attempting to load the web page or fill another request by the browser.
This usually means that the other server is down or not working properly.