This involves Exchange Server 2003 (Enterprise) using an internal IP range of 192.168.100.x.
When building the Exchange server, I wanted to limit connection access to a single IP range outside of the network (a hosted spam filtering service) and to the internal network range. I went to:
SMTP Virtual Server Properties > Access > Connection (Connection Control) Connection
I checked "Only the list below" and first entered an IP range that the hosted provider offered. I wanted to further open access to only our internal range and so I entered "192.168.0.0 (255.255.255.0)". This didn't work as a "blanket range" - instead I had to enter "192.168.100.x (255.255.255.0)".
The reason is probably very simple, but can someone help me understand why 192.168.0.0 would not work? I was assuming that it would envelope 192.168.100.x.