Bri, I'm not sure, maybe follow what essexboy said (both at High level).
For me, when I have a LAN (local network) and a connection to the Internet is shared, the settings must be Medium for the LAN and High for the Internet Zone. Otherwise I can't share components (printer, files, etc.) into my local network (LAN).
Other configurations, I mean, a single computer connected to the Internet could allow both at High level.
I edited my first post.