You can use any range you want on your internal LAN including routable internet address ranges.
But if you do use a publically routable address range that has not been assigned to you there are 2 things you need to be aware of
1) By using 172.15.28.x on your LAN it means you will have problems accessing any 172.15.28.x servers on the Internet. So if the company who has been assigned the 172.15.28.x has a web server you want to access this could be problematic.
2) You must make sure that none of the 172.15.28.x addresses you have used "leak" out onto the Internet. So you would be fine with your DMZ setup because you are presenting your internal 18.104.22.168 address as 22.214.171.124.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...