A friend recently upgraded to a new cable internet connection. The modem connects to the router and various PCs and smartphones from her roommates connect to the router. They don't have any problem accessing the internet.
She has Windows 8 and can't access any website (via wifi and ethernet). DNS (UDP) is working, DHCP set up everything correctly, Wifi is working, Trace routes and Pings (ICMP) go through with no problem at all. But neither Dropbox nor Skype nor Spotify nor any browser (all TCP) can access any website.
The thing is though, she can connect through the university wifi and via a neighbors wifi. It's just her home connection. No firewalls are running and the computer is clean - no malware.
How could it be that only her home connection won't work and others do?