Recently my parents had some friends visit. They had a laptop with them and asked to use the internet. We've not problem with that so let them connect.... except when they connected they were running a peer-to-peer file sharing program.
If you're here, you probably know what that is. A way of sharing large files by turning it into lots of small pieces and allowing all the people who that file to share a little piece with you, until you have the whole file. Then you can share your little pieces with other users.
The advantage of peer-to-peer file sharing is the originators don't need to have and pay for a server with lots of capacity. Used properly it's a great idea. In the past I've downloaded and shared Ubuntu and OpenOffice files this way.
My parents house is connected to the internet via our office. I was trying to set up a server and getting very confused as to why my connection kept dropping. It was making a difficult task impossible. I noticed other things like web pages taking longer than usual, or being sometimes fast and sometimes slow. I check the router and realised what was happening, so I blocked my parents guest completely.
That wasn't enough to solve my problems though. I eventually gave up setting up the server and did it from home in the evening. The problem with torrents is the computers looking to you for little pieces ask for them hours after you've stopped advertising they can have them. This screen shot showed my problem clearly - this was taken AFTER I'd blocked the computer at 10.18.6.56. Essentially they'd invited DoS attack. 11,000 incoming connections was more than enough to ruin our ADSL connection for everyone else on the network.
Sure, we could still email, see web pages slowly, but everything was so much harder than it needed to be.
Moral of the story: If you're a guest with us, please turn off any torrent software before you connect.