IPSec is a network level encryption system that allows you to encrypt any IP traffic on the fly. Its sort of like SSL but much better because it works at the network layer there is no need for you to modify any applications to use it.. anything that can talk over IP can talk over IPSec. This is a big benefit over SSL which requires the application to speak SSL or be enclosed in some sort of wrapper.
The good part about IPSec is that you can very easily establish encrypted communications between two points, such as for a VPN or other remote access solution. The disadvantage is that it requires more processing and generates a little bit more network traffic than a standard IP setup.
There is a simple way to get the MAC address of any machine on your subnet.. simply ping the machine and then run the command
at a DOS prompt. This will show you your system's internal MAC address - IP address table and will list the MAC address of any machine you've recently been talking to. Remember that this will only work on your subnet... since once you leave the subnet, the MAC address becomes quite irrelevant.