A suggestion from Max, the Technical Director & the man who made the engine:
If you are experiencing network related issues (999 ping, red plug icon, cant establish connection to server, authentication issues etc) please run this command in your console (~ key) :
Then after your problem is sufficiently experienced / captured, exit NS2 (DO NOT RUN IT AGAIN OR IT WILL ERASE THE LOG)
Check in your Appdata folder (Windows key + R, type %appdata% then go to Natural Selection 2) and see the now very lengthy log.txt
Copy and paste the entirety of the log to pastebin.com and then put a link to it here, along with the description of your problem and your DXDIAG report (see stickied thread, and please also use pastebin for this as well)
We also need to know if you are using a wireless connection and if you have tried changing the wireless channel? (from 6 to 3 for example)
Thanks for helping! This will really help gather more accurate data on whats occuring for you guys out there having these issues! -Iron
FLOODING / CONNECTION ERROR FREEZES INTERNET & SERVER BROWSER Go to your router's settings (you may need to google how to do this with your particular router model) and change this: Max NAT/Firewall Session Per User : Change from default 1024 to the max 4096
IF YOU ARE USING A THOMSON ROUTER THERE ARE KNOWN ISSUES WITH FLOODING, Please try this fix first below, found on the second page here.
Solution for those with Thomson routers Start --> services.msc Stop SSDP discovery service [SSDP network discovery]--> anwer Yes to the dialog box (to be done after each boot).
Control Panel --> internet/sharing center, "change advancing sharing settings" tab --> Turn off network discovery.
And then, in the router configuration (example: http://192.168.1.254) --> router config menu--> UPnP tab --> deactivateUPnP AV and UPnP IGD. Source