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Ping Vs Arping

Posted on 2014-03-28
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I would like to have a network expert to explain the difference between the commands Ping and Arping, when to use one and not the other.
I also would like to know if Arping is part of windows or Linux OS, or it needs to be downloaded as part of a package.

Thank you
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Question by:jskfan
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by:Emmanuel Adebayo
Emmanuel Adebayo earned 100 total points
ID: 39963686
Arping is a computer software tool that is used to discover hosts on a computer network. The program tests whether a given IP address is in use on the local network, and can get additional information about the device using that address.

The arping tool is analogous in function to ping, which probes hosts using the Internet Control Message Protocol (ICMP). The difference is that while ICMP is a routable protocol that operates at layer 3 of the OSI model, arping operates at the layer 2 (or the link layer of the OSI model) using the Address Resolution Protocol (ARP) for probing hosts. Since ARP is non-routable, this only works for the local network. However, in networks employing repeaters that use proxy ARP, the ARP response may be coming from such proxy hosts and not from the probed target

http://en.wikipedia.org/wiki/Arping

Also, you can get more details from the link below
http://archive09.linux.com/feature/50596

Regards
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by:Don Johnston
ID: 39963777
Ping is a command.
It results in sending an ICMP echo request to the destination specified with the command.  i.e. "ping 192.168.1.2".  In this case the echo request is sent to the host at address 192.168.1.2.  When the host receives the echo request, it will respond with an ICMP echo reply which is sent to the IP address of the host that transmitted the echo request. Basically the whole echo request-echo reply process is an "are you there" exchange.
The ICMP protocol is part of the TCP/IP protocol suite.

ARPing isn't a command, it's a process.
If you are running the IP protocol on an Ethernet network, there are two separate addresses. IP addresses at layer 3 and Ethernet MAC addresses at layer 2. In the previous example, you wanted to determine if the host at 192.168.1.2 is alive. So you used the ping command to send an ICMP echo request.  But that packet has to be carried by an Ethernet frame. Which means an Ethernet MAC address is needed.

So the ARP protocol kicks in and sends out a ARP Request (the ICMP echo request is being held up until the ARP resolution is finished).

ARP request is transmitted to the broadcast destination MAC address (all binary ones). The ARP request basically says "will the owner of 192.168.1.2 please tell me your MAC address".  Because this is sent to the broadcast MAC address, every device on the (local) network will receive and process it. And every device except 192.168.1.2 will end up discarding the packet. But the 192.168.1.2 device will recognize it's own address and it will respond with it's MAC address.

When the originating device receives the response, it will add that information to it's "ARP cache" and finally send the original packet (in this example, the ICMP echo request).

You can see the ARP cache on a windows PC with the command "arp -a"
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by:jskfan
ID: 39963855
what do you mean by arping is a process ?
is it a tool that we 'll have to download in order to use or it comes with the Operating system ?

why should I use Arping instead of Arp ?

Thanks
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by:Don Johnston
Don Johnston earned 200 total points
ID: 39964048
ARP: Address Resolution Protocol.

ARPing is the process of using a protocol (ARP) to determine the correct MAC address for a specific IP address.

So ARP is a protocol. Using that protocol is referred to as "ARPing". Just like a nail is a device used to hold two pieces of wood together. Nailing is using that device.

But unless you're on some weird version of Jeopardy, I don't think anyone is going to be too upset if you say ARPing instead of ARP.

You don't have to download anything. It is part of the IP protocol suite. TCP, UDP, IP, ICMP, ARP are all protocols that are included in the IP protocol suite.  

Simply put, if you can assign an IP address to your PC, you've got TCP, UDP, ICMP, ARP, DNS, DHCP, just to name a few.
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giltjr earned 200 total points
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Actauly there is *nix program "arping", it is part of the IP Utilities package.

The term "arping" can be used to mean what donjohnston is describing, which is the act of sending a ARP out, or can be used to mean the program "arping"

The "ping" command basic function is to use either ICMP or UDP to send IP packet to another host based on IP address.  The host can be in the same or different subnet from the source host.   As with most any IP function the first thing that is done is the IP address is looked up in the route table to see if it is same or different subnet.  Depending which it is an ARP is sent out either to the host or to the next hop address if it needs to be routed.  Then either the UDP or ICMP packet is sent.

Now "arping" can ONLY be used for hosts that are on the same subnet.  It is a very simple program because all it does is send a ARP out for the IP address.  So the source and destination address need to be on the same subnet.  Well it might work if you had multiple IP subnets on the same L2 network.  But it will NOT work through routers.  If you have to route to the destnation IP address, then "arping" will fail.

I am assuming the author of "arping" called it that because that is all the program does, send an arp out.
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by:jskfan
ID: 39968704
giltjr:

Correct , I have seen Arping command used..this is why I asked this question.
you said it is a program. do you know if there is a link to download the package, or if it is Linux package then how to install it ?

Thanks
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by:Don Johnston
Don Johnston earned 200 total points
ID: 39968826
All the arping command does in Unix is that it ARPs the specified local neighbor and creates an entry in the ARP cache (assuming the neighbor responds) for that neighbor.

Without the arping program, the only way to learn the layer 2 MAC address for a neighbor is to try to communicate with the device (pinging it, for example).

The reason that Windows doesn't have such a utility is that there really aren't many occasions where you want to create an entry in you ARP cache for a device but you don't want to actually communicate with that device.

It would appear that the program is installed by default in some distributions. If not, you can download it.

Here's one for Unbuntu.

And here's another.
 
For me, if I want learn the layer 2 address of a neighbor, I ping them.  Then I look in the arp cache for an entry.  If there's no entry for the device, then I know they are not responding to the ARP Request.
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by:giltjr
giltjr earned 200 total points
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--> For me, if I want learn the layer 2 address of a neighbor, I ping them.

Same for me.  I don't remember ever using arping and I personally can't think of a situation that I would use arping instead of just plain ping.
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by:jskfan
ID: 39970126
Many times I when I use ping IP address then type arp -n (if I remember) it does not give me the Mac address of the host, even when in the same subnet (Not beyond the Router)

Sometimes ARP table mapping is wrong too…MAC address to IP address mapping point to 2 different hosts

I am not sure about Arping how efficient it is
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by:Don Johnston
ID: 39970208
If you've got an incorrect or invalid entry in your ARP cache, you can either wait for it to age out or manually delete it.  In Window use the "arp -d <up address>" command.
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by:giltjr
ID: 39971018
Arping uses the same exact method to get the hosts mac address is ping.  It issues a ARP command.  That is the ONLY way to find out what MAC address maps to a given IP address.

--> Sometimes ARP table mapping is wrong too…MAC address to IP address mapping point to 2 different hosts

Just because a MAC address points to two different IP addresses does not mean they point to two different "hosts".  I single "host" (OS/Box) can have multiple IP addresses within the same subnet, and thus a single MAC address will appear for multiple IP addresses.  Examples:   Two L3 switches setup for HSRP or any other device that is setup with a fixed and a floating IP address.


If you ping a host on the same IP subnet and you get a response it will show up in the ARP table.  If it did not, you could not ping it, because ultimately you MUST communicate using the MAC address.
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by:jskfan
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Thank you Guys!
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