Exchange 2013 Admin Console

The way I would do it is first make sure the ip based restriction feature is installed

web server (iis) > web server > security > ip and domain restrictions

then go into the iis manager and select the ip address and domain restriction feature under the ecp folder under what ever site you have it running under (most likely default)

click edit feature settings on the right and switch it to be "deny" by default and the action you want to have for deny (i would go with not found so it looks like its shut off to other people snooping around).  then add the ip ranges of clients you want to allow in the list.

you should have it only accessible from internal networks now (provided you only specified internal ip ranges)

you can try this from www.ers.ie

http://www.ers.ie/Blog/post/How-to-Limit-Access-to-Microsoft-Exchange-2013-Exchange-Control-Panel-(ECP).aspx

Thank you so much for the responses! I actually tried those suggestions on the ECP folder in IIS, and the changes I made also affected OWA. I called Microsoft on it and they stated that was not supported by them. It definitely seems like the above ideas should work. Thanks again!

Thank you! It worked.