ok, I'm confused. I also have thhe DIR825 router and can assign Static IP addresses in two ways.
For example, the router is set to issue IP addresses (DHCP) from 192.168.1.100 to 192.168.1.199.
I can assign any IP address in the range from 192.68.1.2 to 192.168.1.99 to any computer and also enter the default gateway as 192.168.1.1 and the appropriate DNS server IP addresses in each computer. That way those computers have a fixed IP address. I've done this in numerous routers at home and at work, and it works perfectly. On several of my computers and devices, I use the DNS Servers from my ISP, and on a couple of them, including my computer, I use the DNS Server IP's from OpenDNS.com.
The second way would be to do it through the DLink firmware. You would assign a fixed IP address in the DHCP range (in the router firmware) and tell the router what the MAC address is. That way, each time the router sees the MAC address, it assigns the same IP address. You don't have to assign the IP address on the computer that way, just leave it set for DHCP and it will always get the same IP assigned to it's MAC address.
I personally don't see any reason for the second way. I'm using several fixed IP addresses on my network that are below the DHCP range, and they work just fine. I also have an XBox and a Wireless Gaming Adapter, and a Wireless Media Extender and a Windows Home Server, plus my computer that all have fixed IP addresses that are below the DHCP range. They all work just fine with the DIR-825 Router.