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Home Forums ARP (Address Resolution Protocol) explained

This topic contains 60 replies, has 20 voices, and was last updated by   Andrew P 3 months, 1 week ago.

Viewing 15 posts - 1 through 15 (of 60 total)
  • Author
    Posts
  • #15423

    Rama

    Hello,

    Why in the ARP reply packet do we see 00:00:00:00:00:00 as Target MAC address instead of FF:FF:FF:FF:FF:FF ?

    #15428

    Rama

    I meant ARP request

    #15429

    Rene Molenaar
    Keymaster

    FF:FF:FF:FF:FF:FF in binary is all 1s…in other words, it will be broadcasted within the broadcast domain. This way the ARP request reaches all devices in the broadcast domain.

    #15430

    Rama

    I agree with that. That’s why I wonder why we see 00:00:00:00:00:00 is the ARP request screenshot…

    #15431

    virender

    May you pls explain same scenario adding 2 switches and 2 routers in between.

    Computer A ——-Switch1—–ROUTER1——————ROUTER 2 —- Switch2 —– Computer B.

    Much Thanks !!

    #15432

    Rene Molenaar
    Keymaster

    Hi Virender,

    When Computer A wants to reach Computer B it will have to know how to reach its default gateway because both computers are in a different subnet. What happens is that Computer A will do an ARP request for the IP address of Router 1 (its default gateway).

    Computer B will do an ARP request for Router 2 (its default gateway).

    Router 1 and Router 2 will do ARP requests on the link that connects them to discover each others MAC addresses.

    Rene

    #15433

    reza

    thank you.
    why does a pc need a pc*s mac-address that is in same network?if we know its ip address so we dont need to know its mac-address.
    thank you

    #15434

    Rene Molenaar
    Keymaster

    Hi Reza,

    IP is a layer 3 protocol and Ethernet is a layer 2 protocol. If you want to send an IP packet on the LAN you have to put it in an Ethernet frame in order to send it. Ethernet frames use MAC addresses for identification.

    Rene

    #15435

    reza

    thanks Rene

    #15437

    Schweta

    Hello Renne,

    Thank you for all this info on Networking put up in a very simple way.

    Could you please explain what is the difference between ARP Table and Routing Table? And what is Reverse ARP?

    Thanks,
    Schweta.

    #15438

    Rene Molenaar
    Keymaster

    Hi Schweta,

    The ARP table is what you have read about in this tutorial. A routing table is something completely different, best to read this post to understand it:

    http://networklessons.com/ip-routing/introduction-to-routers-and-routing/

    Reverse ARP (RARP) is an ancient protocol…it was used in the past so a host could get an IP address, it has been replaced by BOOTP and DHCP.

    Rene

    #15439

    kiran g
    Member

    Hi Rene ,

    why do we see incomplete MAC entries when a device is connected to a CISCO Router through a switch ?

    #15440

    Rene Molenaar
    Keymaster

    Hi Kiran,

    Which incomplete MAC entries are you referring to? Do you have an example?

    Rene

    #15441

    kiran g
    Member

    Hi Rene ,

    I am referring to incomplete MAC entries connected to the router.

    PNTADD01#sh arp
    Protocol Address Age (min) Hardware Addr Type Interface
    Internet 10.137.21.1 – 0000.0c07.ac01 ARPA GigabitEthernet0/0
    Internet 10.137.21.2 – a493.4cbd.3780 ARPA GigabitEthernet0/0
    Internet 10.137.21.3 0 Incomplete ARPA
    Internet 10.137.21.11 0 Incomplete ARPA
    Internet 10.137.21.12 0 Incomplete ARPA
    Internet 10.137.21.13 0 Incomplete ARPA
    Internet 10.137.21.14 0 Incomplete ARPA
    Internet 10.137.21.15 0 Incomplete ARPA

    #15442

    Rene Molenaar
    Keymaster

    Hi Kiran,

    Ah ok, good question.

    When the router does an ARP request but doesn’t get a ARP reply in return, it will show up as incomplete.

    Try to ping an IP address that isn’t reachable and you’ll see it in this list.

    Rene

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