Solutions to Assignment 5

1. An ARP query is sent in a broadcast frame because the querying host does not which adapter address corresponds to the IP address in question. For the response, the sending node knows the adapter address to which the response should be sent, so there is no need to send a broadcast frame.

2. Data (first 4 rows)/Parity (last row)	Parity bit
   1110	1
   1011	1
   1001	0
   1101	1
   0001	1



3. If we divide 1010101010 0000 by 10011, we get a remainder of 0100, which becomes the CRC.
4. 1. No. E can check the subnet prefix of Host F’s IP address, and then learn that F is on the same LAN. Thus, E will not send the packet to the default router R1.
      Hence, for the Ethernet frame from E to F: source IP=E’s IP address, destination IP=F’s IP address, source MAC=E’s MAC address, destination MAC=F’s MAC address.
   2. No, because they are not on the same LAN. E can find this out by checking B’s IP address.
      Thus, for the Ethernet frame from E to R1: source IP=E’s IP address, destination IP=B’s IP address, source MAC=E’s MAC address, destination MAC= MAC address of R1’s interface connecting to Subnet 3.
   3. Switch S1 will broadcast the Ethernet frame via both its interfaces as the received ARP frame’s destination address is a broadcast address. And it learns that A resides on Subnet 1 which is connected to S1 at the interface connecting to Subnet 1. And, S1 will update its forwarding table to include an entry for Host A.
      Yes, router R1 also receives this ARP request message, but it would not forward the message to Subnet 3.
      B would not send ARP query message asking for A’s MAC address, as this address can be obtained from A’s query message. Once switch S1 receives B’s response message, it will add an entry for host B in its forwarding table, and then forward the received frame as destination host A is on the same interface as host B since A and B are on the same LAN segment.

5. Event	What switch learns	Links on which packet is forwarded	Justification
   B sends a frame to E	switch learns the interface for B's MAC address	A, C, D, E, F	switch does not the MAC address of E yet
   E replies to B	switch learns the interface for E's MAC address	B	switch already knows the inteface for B's MAC address
   A sends a frame to B	swtich leans the interface for A's MAC address	B	switch knows interface for B's MAC address already
   B replies to A	no change to switch's table	A	switch already knows the interaface for A's MAC address



6. False. RTS and CTS are used only for transmission of large frames.
7. Hidden terminal problem: Two hosts attached to the same access point (AP) may be hidden from each other in that they may not realize when their frames collide at the AP.
   If they each send an RTS frame that the AP receives without collision, then the AP can select who to send teh CTS frame to, thus arbitrating to avoid collision. Note that even though RTS frames could suffer from the hidden terminal problem as well, the likelihood is much smaller due to their small size.
8. No, the bogus packet will fail the message integrity check.
9. 1. False
   2. True
   3. True
   4. False

10. Action	Src address	Dest address	Protocol	Src port	Dest port	Flag bit	Check connection
    allow	222.22/16	outside of 222.22/16	TCP	>1023	23	any	no
    allow	outside of 222.22/16	222.22/16	TCP	23	>1023	ACK	yes
    allow	outside of 222.22/16	222.22/16	TCP	>1023	80	any	no
    allow	222.22/16	outside of 222.22/16	TCP	80	>1023	any	no
    deny	all	all	all	all	all	all	no