A: Which ARP and Where

certskills
By certskills October 4, 2015 12:05

Host A web browsers to server E, which sits on a different subnet. Who ARPs for whom, and where? Check the question, pick your answer, and come back here to check your answer and see the explanation.

Letter Answer

B

 

Explanation

All hosts connected to a particular subnet use ARP to discover each other’s MAC addresses. However, for hosts that exist in a different subnet, a host does not use ARP. Instead, that original host sends packets though its default gateway (a router). To learn that default gateway’s MAC address, the original host uses ARP as well.

In other words, hosts use ARP for other hosts and routers in the same subnet.

In this case, because PC A and host E are separated from each other by at least one router, then they must be in different subnets. PC A would not send an ARP request to find server E’s MAC address.

Instead, PC A’s logic is this:

1) Because the destination IP address is on a different subnet, send the packet to my default gateway

2) To learn the MAC address of the default gateway, use an ARP request that lists the IP address of my default gateway, asking to learn that device’s MAC address.

The figure shows only one router, R1, connected to the same LAN as PC A. So, PC A would need to send an ARP request to learn R1’s MAC address on its LAN interface. R1 would then send an ARP reply message back to PC A.

Finally, because PC A’s logic is to send all off-subnet packets to its default gateway, the PC does not need to learn any routes, including a route for server E’s subnet, before sending a packet.

Q: Which ARP and Where
EIGRP Unequal Cost Routes
certskills
By certskills October 4, 2015 12:05
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11 Comments

  1. Alejandro Lanza March 10, 23:35

    First off let me say I enjoy your blogs and videos very much. Now, regarding this topic, i don’t see a topology anywhere, or maybe i’m missing something?

    Reply to this comment
    • certskills March 11, 06:50

      Hi Alejandro,
      Thanks, glad the blogs and videos are helpful!
      Whoops. The figure was deleted. I just added it back. If you don’t see it now, please repost. Thanks!
      Wendell

      Reply to this comment
      • Alejandro Lanza March 13, 16:17

        Thanks for adding it back Wendell, and thanks for having this website here, it’s great to find i have the ability to interact with someone as knowledgeable as yourself in this blog.
        Cheers from Costa Rica!

        Alejandro

        Reply to this comment
  2. AniKk March 25, 03:29

    Hi sir,
    Thank you so much for keeping me glued to this website for a while now, but I am confused about one this, which is PC A must know a route for server E’s subnet before the SUCCESSFUL web connections was made ( I find that to be a logical answer). Can you explain why this isn’t. Thank you once again.

    Reply to this comment
  3. Wendell Odom of Certskills March 28, 07:07

    Hi AniKk,
    Glad you found the posts helpful. I figure that the more I add, the more it’l be useful to folks when they happen to come along and get started with CCENT and CCNA.

    Anyway, to your question. Hosts use 2-part logic. They think:
    1) to send packets to hosts on the same subnet, send them directly. So, use ARP as needed to find the destination hosts’s MAC address.
    2) To send packets to hosts on another subnet, send them to my default gateway aka default router.

    1 and 2 take care of all the host’s forwarding logic. The host does not have a route for any of the remote subnets, such as server E’s subnet. Host A just relies on its default router setting.
    Hope this helps!
    Wendell

    Reply to this comment
  4. Bojan October 7, 09:51

    ARP is one of those protocols that I sort of thought why is it even needed? Why can’t network cards read the IP address, it’s just a bunch of bytes after the MAC address in the data stream anyway, and that won’t eat up that many cycles. Is the use of MAC addresses a legacy, or is there some technical reason why L2 addressing is used today? Like switches could be programmed to switch based on the IP address just as easily as they scan for a MAC…

    Reply to this comment
    • Eduardo September 19, 00:32

      You are aware NICs are Layer 1 devices don’t you? I suggest you first FULLY understand what each layer does then perhaps you’ll understand how important MAC addresses are. Sorry if I sound a bit rude but you must completely dominate networking to understand all its concepts and then it all makes sense – it is beautiful! Good Luck!

      Reply to this comment
  5. Nittney March 5, 01:05

    When A was connected to the network wasn’t there an IP address assigned thru DHCP? and during that process didn’t that trigger the communication between R1 and A resulting in the update of the ARP tables in both even if no one had actually sent anything from A?? Isn’t this part of the boot process for any pc when I’m connected to a network?

    I’m all for trick questions to insure full understanding of a topic, but now we seem to be negating other knowledge to make the trick work. What am I missing?

    Reply to this comment
    • CCENTSkills March 8, 11:19

      Hi Nittney,
      The host may or may not use DHCP, but if it did, I don’t believe the messages in the DORA exchange would actually trigger ARP on the host, given that it’s still not discovered its own IP address nor the default gateway IP address as of yet.

      I don’t think you’re missing anything. The intent is to create a scenario that tests you on a particular idea. Nothing more. It does create an artificial set of conditions for that purpose.

      Reply to this comment
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