Below is an example showing how two Ethernet LAN's may be attached via two encapsulating bridges to a token ring network..
The two bridges must share a common protocol, such as Token Ring. A LAN protocol such as Ethernet can be bridged over another protocol known by two attached bridges. The ethernet frame complete with MAC information is bridged over the Token Ring environment by insertig it into the payload of the Token Ring frame.
This is known as Encapsulated Bridging.