To expound on this idea, the best thing to do is create an org chart that you can fill in of there are consultant brought in. Classically, you will wan to pull together these disjointed sites with a single LDAP directory (eventually if not now maybe later). These directories should mimic your org chart and locations to some logical degree. Your directory structure should mimic this LDAP dir as well. This can help you create role based security, deploy/manage applications, and organize security accordingly.
Failing something sophisticated like that, rely on the OSI model - physical, datalink, network, etc.