*  Exercise
    o Suppose you have two hosts on the same ethernet network:
        + abbott, with IP address 213.172.12.40, 
	  subnet mask 255.255.255.192 and default gateway 213.172.12.1
        + costello, with IP address 213.172.12.21, 
	  subnet mask 255.255.255.224 and default gateway 213.172.12.1

    o Assume both hosts and the router have just been powered on or
      otherwise have had their ARP caches cleared.

    o Describe what happens when abbott sends a packet to costello.

    o Describe what happens when costello sends a packet to abbott.

    o Describe what happens at the router in each of these two scenarios.

    o What subnet mask must the router have on the interface facing
      these two hosts for communication to work?

    o Describe why this might not be an optimal configration for these
      two hosts.