GATE CSE 2019 | Question: 29

Question

Suppose that in an IP-over-Ethernet network, a machine X wishes to find the MAC address of another machine Y  in its subnet. Which one of the following techniques can be used for this?

• A. X sends an ARP request packet to the local gateway’s IP address which then finds the MAC address of Y and sends to X
• B. X sends an ARP request packet to the local gateway’s MAC address which then finds the MAC address of Y and sends to X
• C. X sends an ARP request packet with broadcast MAC address in its local subnet
• D. X sends an ARP request packet with broadcast IP address in its local subnet

Comments

@Nisha Bharti if all the ip address bits are 1 i.e broadcast address and also all bits of MAC address are 1’s i.e broadcast.

then how will the destination know that we are requesting for its mac address as we have to mention the ip address of that particular destination of which we are requesting the MAC address otherwise it will not reach to our required destination and will be broadcasted only.

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@Pranavpurkar it can be a reason.

and i am thinking that if in the place of destination ip as well destination mac ALL 1’s are mentioned then may be all hosts of that network think sender wants my mac address  and that network as it is unicasting to sending the MAC to sender by particular host.

then sender will receiv all mac address of that network.

may this will happen again i am not confirm.

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@Nisha Bharti No Broadcasting can be done without the help of datalink layer & if we want to find mac address of any destination, that destination must also reside in my own network, because routers are act as network boundary.

so, in order to get mac address of destination we need to put all 1’s in destination mac as well as in destination ip address. 

Answer 1

d
that it sends a broadcast request with having IP in local......:o

Comments

Don't remember the option but I marked the same. But, few people are saying broadcast MAC option .

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It asks for MAC from IP, why will it broadcast MAC?

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Ya, that's what the doubt is. Didn't remember if the option was ARP TO broadcast MAC ADDRESS. Or ARP with IP address.

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Google says:
ARP stands for Address Resolution Protocol. When you try to ping an IP address on your local network, say 192.168.1.1, your system has to turn the IP address 192.168.1.1 into a MAC address. ... Because it is a broadcast packet, it is sent to a special MAC address that causes all machines on the network to receive it.

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How does this make the ip answer wrong??

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yes
but not sure that gateway comes in the way ornot

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Can you recall option ? Was it an ARP request to broadcast MAC Address ?

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Were there two options which had broadcast ip address? Or was it only one option?

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two option regarding broadcast ip one its own subnet and other in its gateway

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ARP request will be broadcasted in the same network using mac address FF:FF:FF:FF which is a broadcast address

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No. We say Broadcasting is implemented at Network Layer but in actual, it is being implemented at Data Link Layer using MAC address (which is called Broadcast MAC address - FF:FF:FF:FF:FF:FF). 

Answer is C.

Answer 2

The correct answer is C.

Refer page 467 of Computer Networking: A top down approach, attaching a screenshot here

 

Comments

Option D is correct....

Because the source sends ARP request packet with broadcast IP address to local subnet when the ARP packet is received by adapters then MAC Address

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I think no reference book mentions that arp is sent with broadcast ip address.So answer should be c .

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Some people do not care about references. They only care for their coaching.

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Thats the point sir .

Because they didn't read any std Books.

Only instructor is std for them.

However in both books (kuross and ross and forouzan )this conept clearly mentioned .

Answer 3

According to RFC 826 machine will create a ARP REQUEST Packet as follows: 

HARDWARE TYPE		PROTOCOL TYPE
HLEN	PLEN	OPCODE
SOURCE MAC ADDRESS (X's MAC):(known)
SOURCE IP(X's IP):(known)
TARGET MAC(Y's MAC) : 00:00:00:00:00
TARGET IP(Y's IP):(known)

       

This ARP requests must be sent as broadcasts. There is no way that an ARP request can be sent to the right system. After all, the whole reason for sending an ARP request is that X does not  know the Ethernet address Y . So an Ethernet address of all ones is used, i.e. FF:FF:FF:FF:FF:FF

This ARP Packet will be encapsulated inside a Ethernet frame  and broadcasted. 

FF:FF:FF:FF:FF:FF

X's MAC

TYPE

ARP REQUEST PACKET

CRC

 

By convention, every machine on the Ethernet is required to pay attention to packets with this as an address. So every machine sees every ARP requests. They all look to see whether the request is for their own address. If so, they respond. If not, they could just ignore it.

Answer 4

Ans C) Please refer to the screenshot from Kurose and Ross.