Hey guys, I was having this same issue with my brother and I trying to play together from behind the same router. After a day of troubleshooting, this is the only thing that eventually fixxed things: ... Open steam as admin ... Locate Arma 2, Arma 2 Operation Arrowhead, and Arma 2 Operation Arrowhead Beta. Right click on all 3 and remove local data ... Locate your Arma 2 and Arma 2 OA directories (probably in C:\Program Files (x86)\Steam\steamapps\common) ... Delete the Arma 2 and Arma 2 Operation Arrowhead directories ... Type "Regedit" in search or run window ... Find this registry entry: for 64 bit, HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ (for 32bit: same without the wow6432node) ...Look for Bohemia Interactive and Bohemia Interactive Studio folders ...Click on each entry (so that and ONLY that entry is highlighted), right click and delete them one at a time Great! Now you're ready for a procedural reinstall! ...Close Steam ...Open Steam (right click, run as administrator) ...Go to library ...Install Arma 2 (DO NOT START INSTALLING OA!) ...wait patiently until it's done ...Open Arma 2 through steam, install BattleEye anticheat when it asks, allow game to loadup and exit once you get to the main menu screen. ...Go back to Steam ...Install Arma 2 Operation Arrowhead ...Wait patiently ...Open Arma 2 Operation Arrowhead, install BattleEye again, allow game to loadup and exit once you get to the main menu screen ...Open your web browser ... go here http://www.arma2.com/beta-patch.php ... Download the newest version of the beta (in the green box at the top) ... unzip file, right click on Arma 2 OA beta install file and run as administrator ... Allow install to proceed and finish. ...Open your web browser ... go here http://www.dayzcommander.com/ ... Download the newest version ... Open Day Z Commander ... Click Settings at top ... Make sure "Launch Using Steam" and "Replace original Arma 2OA files with beta so steam works" are BOTH CHECKED ... Click Done and click Install/Update at the top menu ... IT MIGHT SAY SOMETHING LIKE "ARMA2: 100296 (out of date) 99515 (latest)" THAT IS OK, YOU HAVE THE NEWEST VERSION DO NOT TRY TO INSTALL AGAIN THROUGH DAY Z COMMANDER! ... One entry will say "DAYZ (not installed)" ... Use the drop box to locate the latest version install ... Do not install additional maps unless desired, if so install 1 at a time. ... Once install is finished, load up a server through Day Z commander and connect. You should be golden.