Exercise 3 -- Understanding the Internet Protocol

Due: Tuesday, Nov. 17, 1997

1. When we discussed the actual format of ethernet and IP packets, I pointed up that one of their similarities is that they both contain room for the addresses of the computers sending and receiving the packets. I also pointed out, however, that the address formats used by the ethernet and IP protocols were different. Among other things, ethernet addresses are 48 binary digits long while IP addresses are only 32 bits long.

   Any machine that is on an ethernet connected to the Internet has both an IP address and an ethernet address. For example, in class I have shown you both the ethernet address for the Powerbook I use (written as 00:00:C5:D5:D9:09 using base 16 notation) and the IP address for the same machine (written as 137.165.77.66 using what is known as dotted-decimal notation).

   One might expect, therefore, that in an ethernet packet used to send an IP packet, the computer associated with the destination ethernet address included would be the same as the machine associated with the IP destination address specified in the IP packet encapsulated within the ethernet packet. Similarly, one might expect that the ethernet and IP addresses included to identify the source of the packet (i.e. the machine that sent the packet) would identify the same machine.

   In reality, if you could look at the ethernet packets used to encapsulate IP packets on a real ethernet, you would discover that this was not the case. Sometimes, the machines identified by the ethernet and IP addresses used for either the source or the destination or both disagree (and other times they agree). In some cases, in fact, the IP addresses seen would not even refer to machines on the ethernet whose packet's you were examining!

   Explain why these mismatches between ethernet and IP addresses happen (and indeed are essential). In your answer, explain the relationship that would have to exist between the source machine, the destination machine and the network on which the packet was observed for the addresses included to agree or disagree.

If you have questions, you are encouraged to ask them through the discussion area for this homework assignment