I've been having this problem for about 3 weeks now. It started ever since I changed my ISP and even when I use different Wifi networks I still can't host. Everytime I host the game says that my PC is not reachable and I should check if the UDP port 4321 is open in my firewall or router. I've tampered with everything in Windows Firewall trying to 'open' that port and finally I restored the settings to default and downloaded the latest version to see if that will help. The problem still remains and I can't play my Toronto match because my opponent can't host too. Please help ASAP.
Grand Slam - R2(RG '12)
Masters - R3 (IW '12)
TET500 - QF (Valencia '11)
TET250-R2 (Bucharest, Queens '12)
Challenger 125- Q3 (Szczecin '11)
Career High Rank: 75 (05/21/12)
2012 W/L Singles(excluding WO, RL & LR): 5/8 (39%)