This article is from the Token Ring FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
There is no 'converter' that allows an Ethernet network and Token
Ring network to communicate between each other. A conversion process
must occur between the two topologies, since they both use different
signaling types, frame structures, and frame sizes.
There are two methods to accomplish this 'conversion'; bridging, and
Bridging is a method of communicating between devices at OSI layer
2, the data link layer. A bridge connects two networks together and
acts as a traffic director. If traffic is destined to the other
network, the bridge allows the traffic to pass. If the traffic is
local to a single network, the bridge does not pass the traffic
unnecessarily to the other connected network.
The bridge makes this determination based on the Media Access
Control (MAC) address of the workstations on the network. The bridge
keeps an updated list of everyone active on the network, and uses
this list to direct traffic from one network to another.
This method of operation makes the network appear as a single
logical network, since the only separation of traffic from one
network to another is done at the MAC address level.
There are many bridge manufacturers and bridge types on the market.
The newest version of this bridging technology is called a DLC
Switch or LAN Switch. These switches have a much higher port density
than the older two or three port bridges, allowing for much more
flexibility and network segmentation.
The second method of 'converting' from Ethernet to Token Ring is
called routing. Routing occurs at OSI layer 3, and separates
physical networks into separate logical networks. This
differentiates routing from bridging, since bridging maintains a
single logical network.
In a routed network, the sending workstation determines if outgoing
traffic is local or remote. If the traffic belongs to another
network, the originating station sends the frame directly to the
router for further processing.
Upon receiving the frame from the source workstation, the router
examines the frame for the destination address. The router maintains
a routing table which is used to determine the final destination of
the data packet through the router.
Routing is the most common method of connecting Ethernet networks to
Token Ring networks in most organizations. Most network operating
systems have routing capabilities built into the servers. By placing
a token ring and Ethernet card into a Novell NetWare 3.x/4.x or
Windows NT v4.x server, the two topologies can communicate between
One caveat; some protocols are not routeable. A good example is
Microsoft's NetBEUI, which has no OSI layer 3 network address and
therefore cannot be routed. Protocols which cannot be routed must be
bridged between physical networks.