Sharepoint Site - external https access - internal http access

We have an intranet site "Sharepoint - Intranet" - bound to ports 80 and 443.  People can use http or https to access it currently.  I have been asked to remove http access for external users.  The requirement currently is to have the same DNS entry for both sites - so

http://www.intranet.com would be the internal site
https://www.intranet.com would be used externally

I'm unclear on several things.  I have read that one way to accomplish this would be to extend the Sharepoint - Intranet site to use port 443.  But I'm unclear on how/whether this manages people coming in from the outside vs internal and how zones play into it if they do at all.  For example - I extend the site and make it available through https and put it in the Internet zone - then someone types in http://www.intranet.com from external and authenticates that way anyways ...?  Does a redirect also need to happen?

The second way I read that might accomplish this is to route traffic by way of IP address so that internal traffic is allowed to the http site and https traffic is routed to the external site -- as documented in the link below - but I am unclear whether you have to still have 2 seperate IIS sites ...

https://docs.gosecureauth.com/display/docs/URL+Rewrite+-+IP+Restrictions

Or if someone has another suggestion ..

Thanks!