Hi,
About internal websites, their ip address is private. You need to setup a DNS that will be use by your internal users. Your DNS should be configure to host only your own domain (i.e. abc.com) and forward query to your ISP DNS for domains not hosted by your DNS. If your DNS is also public DNS hosting your domain for public query, you can create the following record;
internal.abc.com. IN A 192.168.1.10 << Server internal IP Address
http://www.abc.com. IN A 1.1.1.1 << Server external IP Address (NAT)
For users to connect directly to internal website, use http://internal.abc.com. When they are outside your network, they should use http://www.abc.com
Also, try searching for split-DNS