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Because they are so short, anyone can guess the Metamark urls. Indeed, the mathematically inclined will easily be able to figure out how the ids are allocated! For example a typical url would be http://xrl.us/jx.
If you want to a user from "guessing" your URL, you can add a "privacy code" to your url. If your code is "secret", the url would instead be http://xrl.us/jx-secret.
Then even if someone just randomly tries jx, it won't reveal the real url because it's missing the privacy code.
Of course if you are really concerned about who can see the page it should be password protected or possibly not even on the public internet at all!