Universal Resource

... It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [28], as a location (URL) [29] or name (URN) [30], for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by Internet mail [31] as defined by the Multipurpose Internet Mail Extensions (MIME) [32].

The GET command will accept something of the general form:

  protocol://user:pass@host:port/url

where:
protocol is one of http, ftp, https, etc.
user is a user name,
pass is a password,
host is a host name or IP address,
port is a socket number, and
url conforms to the URL standard [29].

Since HTTP uses TCP as its transport mechanism we can use the TCP program called telnet to connect to the server and request a page. We can, for instance, type:

  telnet www.google.com 80
  GET /

Notice that the GET must be uppercase and the GET line is not echoed by telnet so you are typing blind. What comes back from this request is the default file called index.html. The index.html file is assumed to be the default root page in a web server. So the GET command above is equivalent to:

  GET /index.html

If we look at the first command there are several interesting things of note:

  telnet www.google.com 80

As mentioned the telnet command uses TCP as its protocol which is the same protocol as HTTP uses.

The ``www.google.com'' is a name that gets resolved into an IP address by a program called BIND [38], the Berkeley Internet Name Domain, which is an implementation of the DNS Domain Name System [33,34,35,36,37] protocols.