Ultrapeers are not chosen randomly, - if you have a certain connection speed and a certain average uptime, then you will become an ultrapeer if you try connecting to an ultrapeer that says that it doesn't have enough free leaf slots anymore.

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Morgens ess ich Cornflakes und abends ess ich Brot
Und wenn ich lang genug gelebt hab, dann sterb ich und bin tot

--Fischmob