Why is my Mac rejecting incoming connections?

I'm trying to use a Mac Mini running Snow Leopard to retrieve images from my hospital's DICOM server. My C-FIND requests succeed, but my C-MOVE requests fail, and the DICOM administrator tells me it's because my Mac is refusing incoming connections on port 11112, which is the one that I set up for DICOM communication.
Why is my Mac refusing incoming connections? I have turned off the firewall in System Preferences. I used to have a rather strict firewall set up with ipfw, which only allowed incoming ssh connections on port 22, but I have disabled that too. The Mac is still acting like that firewall is enabled -- I can ssh into it, but I can't ping it. At one point I was trying to use port 104 for DICOM, but I realized that 104 is privileged, so I switched to 11112, and yet that didn't fix the problem either, and I'm still getting the same error message.
What's going on? Do I need to enable one of the "sharing" options?

Not sure it'll help, but you might want to take a look at this: http://support.apple.com/kb/HT2975?viewlocale=en_US

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Why is my Mac rejecting incoming connections?

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