This is a Windows system. As we know, ArcGIS Server creates a default 'arcgis' virtual directory in IIS and that becomes the starting point for various services. While that is a convenient 'one size fits all' kind of a solution it doesn't help trying to secure some web Services which should be only accessible from within the intranet while still enabling other Services to be accessible from the outside.
So what we need is a way to create different virtual directories in IIS for the Services which would need enhanced security. If we could create a different virtual directory for a secure Service then we can simply use IIS's domain/ip based restrictions on that directory to limit to only the intranet IPs. Is that even possible in ArcGIS Servers?
Perhaps, some other option could be to somehow use URL redirect/reverse proxy or use a Service's own security for some role/user based one--but that would require Active Directory on the server and sounds complicating to setup?