This is something any competent administrator should have kept recorded in a database of some sort..
First of all, due to the lack of information about your specific setup, I'll assume you lease IP's in a single range, I honestly don't know how many so let's assume you own an entire /8 range. (CIDR notation)
Now, You have a range of 16777214 IP's and want to allocate them between various departments in your company..
Joe from Marketing requests a small range, you look in your database.. and assign his department 10.0.1.0/25, it's a small department.. only 60 current employees, but they'll grow over time.
Marketing Department:
Net Contact: joe AT marketing.bigcompany.us
Network: 10.0.1.0/25
Range: 10.0.0.1-10.0.1.126 (126 Hosts)
Netmask: 255.255.255.128
Broadcast: 10.0.1.127
Makes sense, right?
