the mac address is on the stickers that came with your modem. Unless your on a router, you can get it by opening the program that runs your connection. The mac address will appear there. Also many routers run mac filtering. Just so you know a MAC addres is given to you, and its permanent, unlike an IP if your's changes. Im sure you knwo the differnece between static and dynamic IP.
The MAC address is embedded in every NIC (Network Interface Card) and is different on every card, and has nothing to do with modems. You can find out what your MAC address is by opening a DOS box and entering "ipconfig /all". Look for "Physical Address" and the number/letter combination to the right will be your card's MAC address (example: Physical Address. . . . . . . . . : 00-03-47-AF-A8-F9)
I believe the question you meant to ask is whether or not you can obtain an IP address when setting your system to use DHCP on a network, and the answer is "Yes." It doesn't matter what the operating system you're using, since any NOS will have support for it. Unless you specifically assign the system an IP address, the default is to use DHCP (well, in Windows, anyway..)
If you're wanting to capture MAC addresses on a network (which was your original question), I'd suggest running Ethereal, Snort, or another IDS, but you probably don't know what I'm talking about so I'll stop there:)
well when i bought all 3 of the modems ive ever had, they all came with a MAC #, i had to put in while setting it up.
Not to start a war here, but I'd like to see a screenshot of that. I've been working with modems for longer than you've been alive and have yet to come across a modem that required anything other than drivers to function. Not to mention the fact that modems aren't used to connect to a LAN.
MAC address as defined by Cisco:
Standardized data link layer address that is required for each port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network, and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, MAC-layer address, or physical address.