Hi all
The system described below is totally closed loop and not connected to a network! So don’t worry about the ip address allocations!
I have a 6.2.1 qnx system with an ip of 128.1.115.98 and a subnet mask of 255.255.0.0. I have this connected with a cross over cable to a windows xp machine with an ip of 128.1.28.1 with a subnet of 255.255.0.0. They ping each other with no problems. I then set the subnet on the qnx system to 255.255.255.0 and i can still ping from the windows system! 
I then change the qnx subnet to 255.0.0.0 and I can still ping from the windows system. I then change the subnet to 255.255.192.0 which should change the allowable range of ip’s to be 128.1.64.1 to 128.1.127.254. I can still ping from the window system!!
I then use ifconfig en0 to change the subnet to 0.0.0.0 to see what happens and qnx ignores this but sets the subnet to 255.0.0.0. Does this mean qnx only supports the first octet of the subnet and ignores the rest? Qnx should see this as a class B and default it to 255.255.0.0 right?
What should I set the subnet too on the qnx system to break the communication link to the windows system?
Thanks