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

What is ARP?

The protocol that helps a host fetch the MAC address of some other host, whose IP address is already known.

 

How does a host use ARP?

Since IP address is already known; first the host uses the Subnet Mask to determine if the other host is in the same network. If yes, broadcast to all. If no, send it to the gateway.

(Options A and B eliminated)

 

How to broadcast? The simple logic is to do so via MAC address; because that's the address the hosts in the same subnet use to communicate with each other. IP address is used for hosts of different subnets (or networks).
We don't know the target MAC address, so in that field pul all 1's. Hence, Option C

 

If we consider Option D true, then the target host would never know the intended packet is for it. Because it won't have it's IP or MAC address.

Answer 2

Initially I was also thinking C is correct but seeing the question correctly I also think D is correct because Arp request packet contain empty or false MAC address source ishttps://www.geeksforgeeks.org/computer-network-arp-reverse-arprarp-inverse-arpinarp-proxy-arp-gratuitous-arp/. And we achieve broad cast  by limited broadcast address.ie 255.255.255.255 (It is used to broad cast with in a LAN) . IT make sense . Broadcast mac is used only if we want to send data within a LAN to all the people .This was flaws of bridge (as per best of my knowledge ) so we used Virtual  LAN concept. Tell me if I am wrong.

Comments

I do not  understand why people see the other resources when Kurose and Ross and furouzan std books gives the ans as Mac address.

Answer 3

Answer is : C

We need to find MAC address that what ARP does,

We have Source Ip, Dest Ip, Source Mac. Then broadcasting Dest IP is of no use. How would destination know that MAC is needed. So MAC bit's set to all 1's. Simply put Dest Ip, source ip and source mac and broadcast MAC. this would send the packet to every stations and whosoever sees their IP would send the ARP reply with their MAC.

Answer 4

step number 3 eliminate option 4.