Using CIDR’s on IIS

Using windows 2000 and running Internet Information services. We have a web site used for internal operations and we would like to make it available to another business. Under “Directory Security”, “IP address and Domain Name Restrictions” I have selected “By Default, all computers will be” Denied Access except those listed.

My Question is how would you use a CIDR in this dialog box? Example x.x.x.x/25