Ethernet and Fast Ethernet use the same frame format. Both use the same frame size (between 64 bytes and 1,518 bytes). Both use the same destination/source address fields, length/type field, variable length data field, and a frame check sequence field. Also both can use the same protocol in exactly the same way.

In addition to the previous features mentioned both types of Ethernets use the same CSMA/CD (carrier sense multiple access with collision detection) algorithm. The MII between the PHY and the MAC can be scaled up from 10bps to 100Mbps without any further consideration.

The main differences between Ethernet and Fast Ethernet besides speed (10bps to 100Mbps) are topology rules. A 10Base-T Ethernet can have several repeaters on its network as long as there are no more than four repeaters between any two nodes on a segment. Where as Fast Ethernet can only have at the most two repeaters on a segment. This can limit the size of a Fast Ethernet network due to attenuation. Some of the solutions to this problem can be overcome with the use of switches and routers coupled with fiber optic cabling to increase the overall size of the network structure.

Another difference between these two types of Ethernets is the signaling methods that Fast Ethernet employs. Even though both can use the same type of cabling, Fast Ethernet uses a different PHY for Category 3, 4, and 5. 10Base-T uses the same PHY for all these types of cabling. The signaling technique that Fast Ethernet uses is incompatible with 10Base-T Ethernet. This also applies to fiber based cabling.

One last consideration concerning the difference between Fast Ethernet and Ethernet is cost. A Fast Ethernet system is more expensive than a 10Base-T system including the cost of the type of repeaters, switches, and routers. This may become a trivial matter in the near future due to the forces of the marketplace and may be one of little consequence.