ARP (Address Resolution Protocol) explained

Hi Alex,

Good to hear you passed the exam. When the receiver receives that stream of bits, it will de-encapsulate it just as if were encapsulated. We don’t skip any layers.

Rene

Hey RM. Can you explain how the switch would be involved in this? If there was a switch where both computers are connected to. Would the switch replicate the ARP request.

Hi Aboubakar,
Assuming that one computer doesn’t already know the MAC address of the other computer (and they are on the same subnet), then the first computer sends an arp request to FF:FF:FF:FF.FF:FF. The switch knows this addresses is a layer 2 broadcast, so it sends the request out all of its ports except the one where it was originally received.

Hi Rene,

ARP, in which layer protocol ?? So far I know , ARP is L2 protocol but it use IP address to resolve the MAC address. Please correct me if any wrong .Thanks

br//
zaman

Mohammad,
You have hit upon a question that has caused great confusion. For example, at Cisco’s forum, there are dozens and dozens of posts about in what layer ARP belongs

https://learningnetwork.cisco.com/thread/36117?start=0&tstart=0

The problem is that the TCP/IP mode and OSI models don’t line up exactly. Personally, I view ARP as being OSI “layer 2.5” because it performs functions in both 2 and 3 (broadcasts at layer 2 based on information from layer 3). By the way, there are other protocols that are equally as confusing (as far as where they fit in). MPLS is a classic example, where most people say it too functions at “layer 2.5.”

sir , for the scenario
Computer A ——-Switch1—–ROUTER1——————ROUTER 2 —- Switch2 —– Computer B.

you said that

"Computer A will do an ARP request for the IP address of Router 1

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."

please rectify/guide me if i am worng
computer A will send ARP request to R1 to know R1 MAC address, so whenever it sends send data to ComputerB it will then send it to MAC address of R1.

sir my second query is that MAC address is required at Local area communication , so why R1 and R1 need to know MAC Address of each other since Router works at L3 (IP).

Hi Muhammad,

Routers do “routing” on L3 but to actually forward an IP packet, they still have to put it in an Ethernet frame. This means they’ll need to insert a source/destination MAC address.

In the routing table, you will find a next hop IP address for each route that it has. If the next hop is directly connected then it will have to do an ARP to figure out the MAC address of the IP address of the other router.

Rene

thanks for reply sir ,
please also guide me if i am wrong , is it right ?
computer A will send ARP request to R1 to know R1 MAC address, so whenever it sends send data to ComputerB it will then send it to R1 MAC address.

That’s correct. ComputerA will have to use its default gateway so it will do an ARP for the IP address of R1.

thanks alot sir ,
your video/tutorial is v useful

19 posts were merged into an existing topic: ARP (Address Resolution Protocol) explained

Hi.

Router A wants to know MAC address of router B. So, it broadcasts ARP. Only router B replies.
In this case, target MAC should be FF:FF:FF:FF:FF:FF which is broadcastin ARP request. Why the target MAC is all 0’s in ARP request?

Hi Ashok,

Take a look at this capture:

ARP Request and reply

The destination MAC address is FF:FF:FF:FF:FF:FF since it is a broadcast. The target MAC address is what we are looking for and unknown, which is why it is empty (00:00:00:00:00:00).

Rene

hi Rene,

can you upload a lesson about isis protocol .

thanks

Lot of times when we ping , we see . !!!
i mean to say the first thing is a . (dot)
is it becoz of our host (router/PC/) trying to find the ARP ?

Abhishek,
You are exactly right! Usually that first time out is the ARP delay.

19 posts were merged into an existing topic: ARP (Address Resolution Protocol) explained

Rene,
Have a small question about the need for ARP or in otherwords, MAC address.

When the packet has already reached the target network, can’t the packet be sent to the recipient based on the unique host id(in that network) alone (the last 8 ip address bits in the case of a /24 network, for example) ? What is need for the concept of MAC address and hence ARP protocol ?

Thanks,
abc

Hi Abc,

Something to keep in mind is that IP is just the “envelope”, IP doesn’t describe anything at all about how to actually send an electric signal on the wire or anything like that. We need other protocols like that…on our LAN, we use Ethernet for this:

Ethernet describes what wires to use, how to send electrical signals etc. Also keep in mind that it doesn’t just carry IP traffic…it could be IPv6 or any other protocols, before IP, that could have been IPX for example.

Besides Ethernet, there’s also other protocols…on our WAN, you might use DSL for example. We use these networks to transport IP but we also don’t use Ethernet (nor MAC addresses) there.

Rene

Ok. Thank you Rene.

Understand that ethernet takes care of the low level packet transfer functions for IP, by interfacing with electrical specifications . And this is needed because there is no other mechanism inbuilt in the electrical systems for packet transfer based on unique IP in the context of LAN networks.

Abc