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Lesson 2HTTP
ObjectiveExplain the Purpose of the HTTP Protocol and common HTTP Error Codes

Purpose of HTTP Protocol and common HTTP Error Codes

What is HTTP?

The Hypertext Transfer Protocol (HTTP) is the underlying communications protocol of the Web and is therefore considered the backbone of the Web. It is the protocol that manages the exchange of resources between clients, namely Web browsers, and servers. Web browsers primarily use HTTP to download documents created in Hypertext Markup Language (HTML); they also use HTTP to transfer files and other data across the network. HTTP is the default protocol for TCP/IP and facilitates the transfer of data into any browser. For example: if you type or if you simply type you will still get the requested Web page displayed in the browser, but HTTP (along with HTML) did all the work.
HTTP defines methods for:
  1. Requesting resources through URLs
  2. Returning error codes to the user

Common Error Codes which appear with HTTP Transactions

There are several common error codes that can appear with HTTP transactions. These codes are included in the HTTP response status line and indicate the outcome of the request. Some of the most common error codes are:
  1. 400 Bad Request: This error indicates that the request was malformed or invalid.
  2. 401 Unauthorized: This error indicates that the request requires authentication or the credentials provided were invalid.
  3. 403 Forbidden: This error indicates that the server has understood the request, but refuses to authorize it. This could be due to insufficient permissions or authentication.
  4. 404 Not Found: This error indicates that the requested resource could not be found on the server.
  5. 405 Method Not Allowed: This error indicates that the requested method is not allowed for the specified resource.
  6. 500 Internal Server Error: This error indicates that an unexpected condition was encountered by the server and it was unable to fulfill the request.
  7. 503 Service Unavailable: This error indicates that the server is temporarily unavailable or unable to handle the request.
  8. 504 Gateway Timeout: This error indicates that the server acting as a gateway or proxy did not receive a timely response from the upstream server.

These error codes can be helpful in diagnosing and troubleshooting issues with HTTP transactions. Developers can use them to identify the source of the problem and take appropriate action to resolve it.

HTTP Error Codes

HTTP error codes indicate the status of HTTP transactions. This lesson does not teach you how to fix the problems, but it does tell you what the error codes mean. Error codes help identify the result of a client request. For example, if you type https:/// and the site does not appear in your browser, there could be an HTTP problem. HTTP uses certain codes to easily pinpoint the problem. Generally, codes starting with 4, for example 4xx, are indicative of a client-side error and codes starting with 5, for example 5xx, are indicative of a server-side error. The table below lists the error codes and their corresponding messages.

HTTP Error Codes
HTTP Error Codes

Determining the category of the problem is important for assessing the problem's severity and priority in fixing it. Once you discover a problem, then it really becomes a matter of whether you are authorized to diagnose it yourself. If you are not authorized to diagnose the problem or if it's a problem affecting multiple users, then you should escalate the problem to the Internet access provider or to the network administrator. In the next lesson, you will learn the purpose of FTP.