People can't join my server.

No one can join my server. I have wanted to make a server and have people join it, but when they try they get a server not respownding thing. I am not behind a router anymore, and my firewall and antiviris stuff is off. I think my modem might be causing the problem. Is that posibly the cause?

hmmm, The firewall of antivirus never were problems setting up a server, turn they back on do not make unsecure your PC, and I never setup a server so sorry