Opening a port by itself is not dangerous in any way. What _may_ be dangerous is if you have a program listening on that port.
Let's take a simple example: Suppose I had a PC running something like DOS 1.0, which is so simple that it has no programs listening to any port. If I open all ports, it would be no more dangerous than having no ports open, because no hacker would get any response from the system - the system is just too dumb.
What is dangerous is the program listening on any open port. Let's say you have a p2p program listening on port 32658. If the world was perfect, I would be able to connect to that program and get the files I was authorized to get, and that was all. But in the real world there may be bugs in that program, so if I send it too many requests, or send it a request for an invalid file name, it might cause the buggy program to do something unexpected, such as send me your password file, or open up other ports, etc.
In other words, open ports are dangerous to the extent that the programs you have listening on those ports may not be trustworthy. If you trust the program then you can feel safe. However, any time you open ports there is that slight risk, so you want to take precautions like configure the program to do the least possible, make sure you apply updates regularly, backup your important files etc. P2P programs in general are not as trustworthy as well known and well tested programs like web servers and email servers.
HTH.
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by: EmpKentPosted on 2007-04-28 at 20:20:24ID: 18996072
All ports can be used for any service but if no service is active, the PC will respond with a not available message unless the PC is firewalled on that port and then it will not respond at all. If you have a hardware firewall and that port is not open, the firewall will drop the packet before it ever gets to the PC.
You do not use port 80 to browse the Internet, you would use it to host a webpage so unless you are hosting services, like HTTP or e-mail, you do not need to have any ports open.
Kent