Although a namespace usually looks like a URL, that doesn't mean that one must be connected to the Internet to actually declare and use namespaces. Rather, the namespace is intended to serve as a virtual "container" for vocabulary and undisplayed content that can be shared in the Internet space. In the Internet space URLs are unique, hence you would usually choose to use URLs to uniquely identify namespaces. Typing the namespace URL in a browser doesn't mean it would show all the elements and attributes in that namespace; it's just a concept.
Note that the address used to identify the namespace, is not used by the parser to look up information. The only purpose is to give the namespace a unique name. However, very often companies use the namespace as a pointer to real Web page containing information about the namespace.
The namespace attribute is placed in the start tag of an element. A namespace prefix is considered in-scope on the declaration element as well as on any of its descendant elements. Once declared, the prefix can be used in front of any element or attribute name separated by a colon (such as s:student).