An active monitor is the device in a token ring  that performs network-management duties, such as keeping track of tokens and weeding out frames that would otherwise circulate indefinitely. The device that has the highest MAC address

MAC address

A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...

in the token ring (the first device online) is automatically selected as the active monitor.