It is usually not possible for a person to get the MAC address of a computer from its IP address alone. These two addresses originate from different sources. Simply stated, a computer's own hardware configuration determines its MAC address while the configuration of the network it is connected to determines its IP address.
However, computers connected to the same TCP/IP local network can determine each other's MAC addresses. The technology called ARP - Address Resolution Protocol included with TCP/IP makes it possible. Using ARP, each computer maintains a list of both IP and MAC addresses for each device it has recently communicated with.
Most computers allow you to see the list of IP and MAC addresses that ARP has collected there. In Windows, Linux and other operating systems, the command line utility "arp" shows this information. Using "arp," you can in fact determine the MAC address of some computers from their IP address. ARP works only within the small group of computers on a local area network (LAN), though, not across the Internet. ARP is intended for use by system administrators and is not generally useful as a way to track down computers and people on the Internet.