I'm afraid I didn't even ask where the modem is (it's not in my house, that's for sure). My ISP says I'm connected through a cable which goes for 70 meters outside my house where it enters a switch. It seems this switch collects several computers in the neibourhood, connecting them afterwards to a server running linux. I was told to set this as DNS and gateway (220.127.116.11). The connection of this server outside is through optic fibre.
I do have internet connection, the setup seems ok from this point of view. I also asked if the ports are open, and he said that in principle, they are all open. Of course, he advises me to have a firewall for my own security.
I do have panda dealing with it, and it happened to notice once in a while a message saying that IIS is trying to do something potentially dangerous. I allowed it to.
But when I ask trace route to get to my address it does not get to my gateway (shouldn't it?). It stops somewhere at 18.104.22.168/22.214.171.124 (which belongs to RDS, my provider's provider). I don't get it: whatever ports I have open or closed, shouldn't it get to my gateway anyway first? I really want an answer to this question, please, in order to understand this strange connection.
Can I use under this conditions my personal computer as a personal web server?
My ISP says I definitely am, and the problem is in my computer. Where? As far as my computer is concerned, I can access my ports (80, 81, 8000 etc). What more can I check?