Lets say it's like a wireless network, if it is secured by automatic encryption, you type in a pass key, and it generates a hex key for the network identification. MAC addresses work just like this, except not security drawn... The dhcp server (in this case i'm guessing a router) will read the MAC and produce an IP address. if you change the MAC address of your network card/computer, your ip will change, it's just how DHCP servers do their thing. If you don't want this, you can just access your router (usually 192.168.1.1), and tell it to turn off dhcp. You will of course have to manually enter the ip data on your comps, but by entering 192 ip addresses, the router will accept clients, and your ip will never change.