If you overlook my brain-dead mistake then along with the observations from The_Berzerker and Arani you've got a fair picture. If you want to break your enterprise up then either subnetting or VLANs (which implies subnetting anyway) will do it for you. Work out on paper exactly what you need to do, with the IP ranges and subnet masks to achieve it. And finally, don't forget that when you've separated everything you need to enable the subnets or VLANS to communicate so you'll need routing between them. The basic rule of thumb is to divide up the nodes such that you minimise the need for intercommunication; however you won't eliminate it completely so you need to allow for it
