why do you need to separate your network? 18 users will not affect your network performance . having separate network will only give you another problem...imagine that all users are connected only to 1 router and 1 switch. if you separate the two depts..they cannot see each other.and file and printer sharing for all PCs will not work as well as internet connection for one subnet. except if you will use special router like Cisco to route your network A to network B inorder to see each other.