It is configurable but I have had problems connecting to hosts with a x.x.x.0 or a x.x.x.255 address from Windows. I have some routers each configured with a /32 host address on Loopback0. I could reach them all but when I configured one with 192.168.250.255/32 my Windows PC couldn't reach it. If I made this .254 it worked. I think this was an issue with the Windows IP stack since it is perfectly legal if you are subnetting all the way down to /32 host networks.
HTH
Andy
