The link goes to the middle of a thread if it has the id number of the posting attached. If you look at the url there is
the protocol used: "http:"
the website: "www.mudcat.org"
the page: "thread.cfm"
the thread: "threadid=4291"
and finally the posting id: "23430"
(of course there are some characters in between to mark where which part starts).
HTH,
MudGuard