I have a crack in a bathroom sink drainpipe. The piece that is cracked is the piece that attaches on one end to the P-trap, and on the other end to the wall opening. I have removed the P-trap successfully. Now, I want to remove the drainpipe attached to the wall stubout. The drainpipe looks like it connects directly into the large, round brown pipe/opening that goes into the wall. I tried to unscrew the drainpipe, but it will not budge. I am worried that if I pull too hard that I will break something inside the wall, or mess up the threads that I believe exist in the round, brown opening. Will I be able to remove the drainpipe by unscrewing it (I am trying to use a pipe wrench to hold the rounded drainpipe with little success), or is there some unscrewing of the round, brown opening involved ?