stuey versus the quizzes, Episode 4: Multicasting

[CriscoSystems]

Once again, I am fighting with the answer keys in one of my BSCI books. Here are the questions I struggly with; I won't offer my answers nor the answer-key answers; I want to hear everyone's interpretations first.

1. Multicasting is efficient in terms of processor resources for which of the following reasons?

A. Multicasts can be ignored by the network card.

B. Multicasts have a specific set of IP addresses.

C. Multicasts are not routable.

D. Multicasts do not support TCP.

2. Which one of the following multicast addresses does not share a multicast MAC address?

A. 225.145.1.1

B. 227.17.1.1

C. 235.17.1.1

D. 237.117.1.1

3. Which of the following is not a capability of of UDP multicast?

A. Packet ordering

B. Retransmission

C. Limiting receivers

D. Dynamic membership

4. Multicasting supports applications that communicate

A. one-to-one

B. one-to-many

C. many-to-many

D. many-to-one

[Giuseppe Larosa]

Hello Stuey,

I try to give my answers below

1)

A Multicasts can be ignored by the network card.

on a router sh ip interface typex/y shows what multicast addresses the interface has joined

includes 224.0.0.1, 224.0.0.2, well known multicast addresses associated to routing protocols

2) here the question that only the last 23 bits are mapped to the L2 MAC multicast leaving 5 bits not mapped

first 4 bits of first byte are 1110 fixed and don't need to be mapped

1110 0000 = 224

third byte msb doesn't count so

145 is like 145-128 = 17

answer D

3)

A and B

no explanations needed here clealrly reordering and Retransmission are not UDP features (also for unicast)

4)

answers B and C

Hope to help

Giuseppe

[CriscoSystems]

Thanks Giuseppe - right after I posted I did some reading so the answers to 3 and 4 don't bother me anymore.

Number 2 is still a mystery to me - all the addresses listed are valid multicast IP addresses; and each multicast IP address maps to a MAC address. Yes, it shares the address with 32 other IP addresses; but EVERY multicast IP address does that; so I'm not even sure what the question is asking.

[viyuan700 · Accepted solution]

Number 2 is still a mystery to me - all the addresses listed are valid multicast IP addresses

Binary for all your address is

225.145.1.1

227.17.1.1

235.17.1.1

237.117.1.1

11100001.10001001.00000001.00000001

11100011.00010001.00000001.00000001

11101011.00010001.00000001.00000001

11101101.01110101.00000001.00000001

Since only last 23 digits are mapped to multicast address. You can see that address A, B,C will have the same multicast address though the IP address is differnt.so as per question they share the address.

Only D is different mulicast address. As Giuseppe noted D is the answer.

[CriscoSystems]

Ah yes, I see now.

This was one of those "misplaced binary digits" problems.

[mlund]

Hi

I agree with what Giuseppe says, but how about

1,B. Multicasts have a specific set of IP addresses.

Multicast uses class D, 224.0.0.0-239.255.255.255

For me this is a specific set of addressess.

What do You guys think.

/Mikael

[Peter Paluch]

Hi Mikael,

You are completely right about the specific set of IP addresses reserved for IP Multicast. However, the question asked about what are the reasons that the IP multicast is efficient in terms of processor resources. In my opinion, it is somewhat questionable how a separate IP range conserves the processor resources.

Best regards,

Peter

[mlund]

Hi Peter

I missed that. That is probably why I always have problem with the written tests, either I read the questions to quick, and missed some words,or misunderstand them, or when I have read the qustion and starts reading the alternatives, I forgot what was the qustion.

It is always nice to read Your answers in the forum, they are always detailed and well described.

/Mikael