By default D-Link routers start assigning DHCP addresses at 192.168.0.100 and going up, at least on my DI-524. How would an internal IP address affect security?
The 192.168.X.X addresses are used because they denote a private network. The reason you cannot assign your home network to another IP range is; Suppose you want to visit a website "http://www.somewebsite.com" and its IP Address is 172.120.1.1, and you share the same IP address, your computer is going to direct you to port 80 (Web Traffic) on your home computer. Which in most cases a webpage is not going to be displayed and you are not going to be able to visit "www.somewebsite.com". Hope this helps, have a good day.
most are automatically set because 192.168.x.x is a reserved private ip address space.
as far as secuity... you can enable mac address filtering, limit amount of dhcp leases and you could also disable broadcasting of your ssid. You could also set static ip addresses on all of your computers connecting to this router and disable dhcp all together.