Mac can't connect to Internet, no IP, no DNS even with iPhone tethering

Question

My Mac Mini Server with 10.6.8 Snow Leopard Server doesn't connect to the Internet.
It can connect to the router either via Ethernet or Airport, but it can not get an IP address -- it always assigns itself a random IP address which is not in the correct range for the router.
All other devices, including PC, iPhone and iPad can work correctly with the router.
When I manually typed in the router's IP in Safari, 192.168.0.1, I can see the router admin page. But typing in any external IP nothing shows up.
Thinking it might be the router's problem, I used iPhone 4S' tethering -- tried wifi, USB, Bluetooth tethering and the Mac Mini still doesn't have any Internet connection.
I changed the DNS in the Network preferences, trying Google's public DNS 8.8.8.8, my ISP's DNS, and empty DNS. No luck.
What steps should I take to diagnose this problem further? Coming from a Windows background I'm not sure which tools are available to solve this problem...

Answer 1

The answer does not vary.  (See details below for why this is the case.)
Check your local DNS on the server; more likely than not, there's either an IP routing error around the gateway setting, or there's a problem with the local DNS set-up (if you're using DNS to reference the remote systems.)
Launch Terminal.app (from the Applications > Utilities folder) and issue the command
sudo changeip -checkhostname
And see whether that produces some output and indicates DNS is valid and no changes are needed, or (as I'd suspect) that there are DNS errors here and that DNS or host-level changes are needed.  You'll need to specify an administrative password for the sudo command.
Also check the IP gateway setting in the server.
And (again) you need to use a static IP address for this box, within the same subnet you are using on your network.  This address should be outside of the DHCP address pool(s) in use on your network.
---- more details ----
Irrespective of your usage, this box is not a desktop (client) machine.  It's a server.
Server requirements do differ from clients.
Servers expect (at least for themselves) DNS services, and require a static IP address.
Servers also support more than one user, which means that server outages can effect more than one user.  This detail can then lead to differences from how client boxes are typically managed.  Server administrators tend to make fewer changes to and to install and reconfigure a server, and can variously make more and can make more specialized backups, than would be the case with a client box.
To use this Mac Mini Server as a client box, it's possible (per other discussions posted here in the forums) to load OS X Client onto most (all?) Mac Mini Server boxes.  Caveat: I've not tried this, and I don't know off-hand if this is officially supported by Apple.
My general preference involves getting DNS services going on the local network, and to use the server for what it provides.  Having DNS services on the local network can provide value, as do various other capabilities provided by OS X Server.  And to treat the box like a server, with static IP and proper DNS services and related.  My preference is to use a server as, well, a server, and to serve various protocols to the local clients.  Because of this, servers are useful, but they're also inherently somewhat more involved to configure than are clients.