Segmentation & DHCP

Sub-netting is not the easiest thing to grasp. I have read alot, but many questions still.

Imagine you have 5 departments with 20 clients each. If you segment it (ex. 255.255.255.224), then you have to use fixed IPs correct?

This could mean a lot of work if you have to change machines or re-install software often. Even worse, if the clients are connecting to a Wlan and you have 50 users but only 20 can connect at a time.

: Is it possible to have a segmented network with DHCP?

I am not sure I understood your question completelly, but you can use DCHP, as long as as you have a DCHP server, or, perhaps more efficiently, a DCHP Relay agent on each subnet that will relay the DCHP broadcasts at your main DCHP server.

A picture=1000 words, but I don't have one. But I think you're on the right track with your answer, and I will look into the DHCP raly agent.

In this particular hypotetical network, the DHCP server is an Internet router. If we use dynamic IP addresses for the clients, then how does the router know what IP to assign a particular client that should be in segment A and not in segment B? This is where i'm getting confused!
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If the router next to the client supports BOOTP, it will fill the giaddr field on the DCHP message with his proper interface address, before forwarding it, so the dchp server based on this can provide a configuration from the proper pool and also know the address of the relay agent to whom it will unicast the reply. The exact mechanism is described in rfc1542