Ethernet Details

Ethernet Frame details

An ethernet frame consists of

• 8 bytes of preamble (all 010101 for the hardware)
• 6 bytes dest address (the MAC)
• 6 bytes source address (also a MAC)
• 2 bytes frame type (either an IP type or others like IPX)
• 64 - 1500 bytes data (note there is a min length)
• 4 bytes CRC

Addressing

All ethernet cards have a built in phyical address (sometimes called the MAC address).  Changing cards on a host changes the MAC.  Remember that all ethernet cards hear all packets sent on the net.  However, cards only process packets with addresses that are either 1) the cards's MAC, 2) the network broadcast address, or 3) any extra addresses given by the operating system as part of a multicast.  Cards can also be put in promiscious mode.

Extending an Ethernet

Repeaters are electrical amplifiers and filters.  They have no data capabilities.  For 10Mb or 100Mb ethernet, you may have only two repeaters can be used on a segment.  They are not used in practice.   There is also a maximum wire length that can be used in ethernet.  For 10Mb ethernet, it's 2.5km,  for 100Mb, it varries from 400 meters to 205 meters, depending on type of wire and other things.

Bridges are computers with multiple interfaces.  Each wire connected to one interface is called a segment.  When a packet arrives in interface i1, that address is added to the list for interface i1.  The hard part is to decide where to send this incomming packet.
Also, if the destination for the packet is not on the list for i1, then the packet is repeated to the other intefaces.

1. First, add the source address to the list for the interface the packet just arrived on.
2. If it's a broadcast and multicast addresses, send it everywhere.
3. If the dest address can be found on the list for any interface except the interface it came on, send it to that interface.  In otherwords if a packet from interface i1 has a dest adress that can be found on the list for i3, send it to i3.
4. If the dest address can be found on the list for the interface it came from, do nothing.  In other words, if the packet from interface i1 has a dest address that can be found on the list for i1, drop the packet.
5. If none of the above, send it down every interface.

In a complex network with multiple bridges, the bridges must be careful not to create cycles of repeating packets.  Any such cycle can flood the network.  Also, bridges should take care to learn the best route from any segment to another segment.

Often bridges filter packets on various criteria.

A bridge is just like a router for only one protocol.