For high traffic scenarios this message shows a problem on the workstation running Webserver Stress Tool. The IP stack can't create additional outgoing TCP/IP connections because it runs out of available socket numbers for the IP address(es) that you have configured in Webstress.
This error occurs when your workstation cannot allocate new sockets. On Windows the default limitation is 5000 - 1024 open sockets (5000 is the default max user port setting and the first 1024 ports are reserved).
You can increase the default maximum socket value in the registry:
HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Tcpip/Parameters
and add or modify the "MaxUserPort" value.
See:
support.microsoft.com/default.aspx?scid=kb;en-us;196271
Also this "Cable Guy" article from MS TechNet:
www.microsoft.com/technet/community/columns/cableguy/cg1205.mspx (English)
www.microsoft.com/germany/technet/datenbank/articles/600997.mspx (German)