Question : How deos DHCP Work

Hi All.  When trying to diagnose some of the many problems we have with dhcp and configuration, we always get stuck becuase we dont know basically how it works.

Could someone please explain it.

For instance, how does a windows 2000 client know when set to use dhcp, where the dhcp server actually is.  Is this controlled  by the wins, perhaps.

What actually happens when a client is set to use dhcp,  does the server constantly ask "who is out there" or does the client say "who is going to look after me" and what actually happens then?

Answer : How deos DHCP Work

DHCP is a pretty nifty I must say.
Here's what happens...

Some client (lets say this is a desktop connected to a switch with a DHCP server and a router)
The client plugs in the patch cable to the switch, windows (in this case) recognizes this and wants to send a 'broadcast' on UDP Port 53 saying I need an IP address!
The server acknowleges this and assigns an IP address from a list which is defined in server configuration files. There are files in the DHCP server which say who has what IP address, so there arn't any 'collisions'.
In linux its usually in a file called dhcpd.conf in your /etc directory, in windows server its accessible by the microsoft management console (mmc) under the dns server snap-in.

So.. how does this windows 2000 box know which DHCP server to talk to? Problem is, it doesnt. This is the real scare with something called a 'rouge dhcp server'. This is when someone sets up their own DHCP server (apart from the networks) and begin distributing IP's from that computer instead of the main one (it's a first come first serve, so whoever's DHCP packet gets there first is the IP it gets) Generally there should be only one DHCP server per network. If you have more than one network, or a branch of another network you can setup something called DHCP forwarding where the router in the second network acts as a middle-man connection between the master DHCP server and the client. You can also turn that second router into a DHCP server of its own if you'd like.

WINS (Windows Internet Name Service) is a different thing from DHCP. In the DHCP server you can set which servers are your WINS servers, but WINS is mostly just for resolving names like \\computername to IP's.

In DHCP, you also define what IP address your router is. That way, clients know what server to use for their internet connection. You also set the DNS servers in DHCP so clients know what IP address belongs to google.com