Packet headers
         Ether         IP          TCP         Data
      Edst Esrc T
                      IPsrc IPdst
       Network devices may use {ether, IP} headers to do it’s job
       (sometimes in twisted ways)
                                   
           Types of ethernet addresses
      • Use ethernet addresses to be able to filter 
         packets in ethernet receiver hardware 
         (performance)
      • 3 types of addresses:
          – Unicast: single host
          – Multicast: group of hosts
          – Broadcast: all hosts
          – (tell by lsb of first byte of address)
                                   
                   Collisions (1)
     • Ethernet is CSMA/CD (Carrier Sense 
       Multiple Access/Collision Detection)
     • Listen before you transmit
     • Listen while you transmit
                             
                      Collisions (2)
      • If there is a collision, both transmitters will:
         – Jam
         – Backoff (exponentially, randomly)
         – Re-try sending the packet
      • Minimal packet size: 64 bytes
      • Collisions are harmless!
         – Actually help scale / stabilize an ethernet network
                                  
           Thick ethernet (10base5)