How can i set my subnet mask to 255.255.255.255 in windows..
without it telling me its invalid??

You can't... because it's not valid. Why would you want to do this anyways?

set it where? Your system TCP settings or in your router?

the most common mask is 255.255.255.0

My TCP settings ..
And FYI.. i think it is highly possible..

If you use a subnet maks of 255.255.255.255, you will not be able to access any other network device. This subnet mask basically creates a subnet with only 1 valid IP address in the range (the IP of the device it's on).

which is valid for the system nic's address, but not on a device which has more traffic,
eg a router.

see http://support.microsoft.com/kb/164015

Ok.. I knew all of that.

What i want to know is .. How to do it in windows without it telling me its invalid.

That article doesnt help jobeard..

The only way windows will let you have a subnet mask of 255.255.255.255 is to edit it within the registry. HKLM, system, currentcontrolset, services, tcpip, parameters. Then find the interface that corresponds to you NIC. As always, edit the registry at your own risk. I still don't see the relevance in doing this, but you clearly have your reasons.

there is Broadcast traffic at the x.x.x.255 address for several ports which would be missed with your submask setting.

you could easily send anywhere but only traffic specifically addressed to your IP could
be seen.

this would be a very extreme measure to control bandwidth.

Very interested in your goal(s) on this choice.

btw: I disdain software that is parochial too

article on subnet masks

Scroll to "Apply a Subnet Mask"