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 http://www.excite.com or if you simply type www.excite.com 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

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 http:///www.yahoo.com 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.

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.