Another possible (but somewhat less elegant and it'd probably be a headache to enter 300 addresses) option would be to do something like this:

1. Open System Preferences
2. Select Network preference pane
3. Select your network interface (e.g Ethernet). If you have multiple interfaces active, yes, you do the rest of the steps multiple times.
4. Click the button that says "Advanced ..."
5. Select the tab that says "Proxies"
6. Tick the checkboxes next to "Web Proxy (HTTP)" and "Secure Web Proxy (HTTPS)". For both cases, set the Web Proxy Server to something like 127.0.0.1 (aka loopback)
7. In the section labeled "Bypass proxy settings for these Hosts & Domains", enter the addresses you want to whitelist in a comma-separated list, I think.

For bonus points, if you're bored, do the following (unless you have your Firewall set to block everything in which case it might not work):
1. Go to System Preferences > Sharing
2. Enable Web Sharing
3. Write up a scolding message in html, name the file "index.html" and put it in /Library/WebServer/Documents/