Subnetting Question

[Scott Mecham]

Need help with subnetting.

My company owns a half of a class C license of ip's. For example I'll 
call the network 10.5.73.0/24. But we are only allocated the upper 
portion of the class C ip range (10.5.73.128-10.5.73.255).

How do I mask these out to my router and then to my firewall? How do I 
mask out only the upper part of the ip range?

I've tried 10.5.73.128/25 or 255.255.255.128. But it does not work properly.

We have a T1 internet line. Then a Router. Then a firewall wich provides 
NAT. Then our LAN (192.168.0.0/24).