Every TCP/UDP endpoint (such as a client or a server) maintains a state-table. As you might guess, it stores the state of each connection. You can view it using

Every TCP/UDP endpoint (such as a client or a server) maintains a state-table. As you might guess, it stores the state of each connection. You can view it using the 'netstat' command on Windows and Linux. Bittorrent tends to create several entries in the state-table (since you're typically connected to hundred of seeds/leeches), which are each stored in memory, and uses up CPU resources to setup/maintain/teardown. So if you were downloading a file via, say, Rapidshare, there'd be probably 1 TCP (you can configure your browser to use more) connection for your router to track, and do the necessary TCP handshakes, acknowledgments, RWIN scaling etc. Multiply all that's necessary for 1 TCP connection by N, where N is the number of peers you're connected to, and the limited resources on a $60 router get used up fast. Early models of the now-famous Linksys WRT54G could reliably be crashed by torrenting while using stock firmware. A power-cycle was necessary to get it running again. Fun times. However, all this only applies if you're running the router in NAT mode, which means the router takes the IP address from your ISP, and acts as the endpoint on behalf of your computer. If the router is running in bridged mode, then your computer acts as the endpoint, as much more equipped to handle large numbers of TCP connections. Edit: Formatting, Grammar.