Every network adapter has what is called a Media Access Control address that uniquely identifies it. Think of it as an ID. Having to manually go to every computer on the network, and taking the time to run tools to find out every device's MAC address is extremely time-consuming. And even then, it may not work, because people may have connected tablets or smartphones to the network, and you may not know every person who has done that. There's an easy way to do it : Get LizardSystems's Find MAC Address ($30 for business use, 30-day free trial; free for non-commercial use).
Simply tell the program the range of IP addresses on your network, click a button, and it looks at every address, ferreting out the MAC address for any device using that IP address. In my tests, Find MAC Address did a stellar job and was able to find the correct MAC address for every device connected to my network: a wireless router, multiple PCs, a Macbook, a home server, and even a Xoom Android tablet. Based on the MAC addresses, it was also able to accurately identify the right manufacturer for each device.
Find MAC Address includes some very nifty tools, such as the ability to look at a specific IP address and identify the MAC address of network cards. There's also some settings that only techies will be able to fathom, such as determining the method of checking a computer's remote state--but you may not need to use such complex settings.
If you run a network, you know how important is that you know the MAC address of every device connected to your network. You may want to know this security purposes, so that you can tell if any unauthorized devices have managed to worm their way into your network. And you also need to know the manufacturer of every network adapter for support reasons.
If you administrate a network and need to find out the MAC address of every device on it, Find MAC Address is a great tool. But at $30 for business users, you won't want to pay for mere curiosity. That said, it's free for non-commercial use, so if you run a network for a nonprofit or an educational institution, the price won't stand in your way.