That's not URL. Understand the Concept. You having some Basic problem. Use that id as Parameter(GET method).
See this Image:
Get something? For more...
As your point use
$this->uri->segment(2);
As your question,
localhost/codeigniter/ripon
this is your URL. So, in your default Controller, way 1:
public function index($user)
{
echo "User is ".$user;
}
way 2:
public function index()
{
$user = $this->uri->segment(2);
echo "User is ".$user;
}