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How does the ping command really work? Specifically where does the ARP (Address Resolution Protocol) come into picture?

I was asked this question in an interview and I was not able to come up with a scenario when ARP could be used.

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migrated from stackoverflow.com May 3 '11 at 12:56

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Look into ICMP. –  Mikaveli May 3 '11 at 12:40
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This is not a programming question. –  unwind May 3 '11 at 12:41
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ARP is used to get the MAC address of a specific IP address. When you need to send a packet on ethernet you need the MAC address of the destination. –  Ankur May 3 '11 at 12:42

5 Answers 5

up vote 4 down vote accepted

If you really want to understand, there is an excellent (very well written) white paper here:

http://images.globalknowledge.com/wwwimages/whitepaperpdf/WP_Mays_Ping.pdf

Here is the summary ->

Ping (Program on the application layer) ------->
Opens a 'raw' socket to IP Layer ------>
IP layer (Layer 2 on OSI) packages ICMP packet and sends it

Since there is no TCP layer in between, the Ping (program) has to monitor all the incoming ICMP packets and filter only the one's from the destination.

Hope that helps.

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Ping is actually two different ICMP (Internet Control Message Protocol) packets.

To ping a host you first send a ICMP Echo Request Packet, the host will then reply with an ICMP Echo Reply.

For more information see: http://en.wikipedia.org/wiki/Ping

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+1 for mention ICMP –  0verbose May 3 '11 at 12:47

Ping and ARP are different things located at different layers in the network protocol stack.

Ping is at network layer (or Internet layer - Have a look to ICMP protocol like pointed out by @ServerMonkey).

Arp protocol is at link level (a lower level). Arp protocol is designed to allow physical connection between network hardware, that is directly connected.

In TCP/IP network stack, every layer uses the layer below to forward its data, encapsulating it inside the low level protocol. Each layer is independent from the other and possibly unaware of the other levels specific details and implementations (this is not always true: see cross-layer function).

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yes.but ping implementation would require some code at L2 (link layer.).does ARP come into play at Layer 2. –  liv2hak May 3 '11 at 12:43
    
@liv2hak. no. ping implementation lay on the below link layer. –  0verbose May 3 '11 at 12:45

ARP provides a MAC address, but sometimes if there is no DMAC address, the broadcast address is used.

This frame using broadcast DMAC is called as ARP broadcast frame, with this we get DMAC address.

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Assuming the ping involves a packet being sent over an Ethernet or WiFi network, ARP is used to find the Ethernet hardware address of the device that receives the outbound packet. Typically this will be the router for the LAN the machine originating the ping is on.

The typical process is:

  1. You enter a command to ping a destination.

  2. DNS is used to determine the IP address (if needed).

  3. The routing table is consulted to find the next hop towards that destination.

  4. ARP is used to find the hardware address of the next hop.

  5. The IP packet is sent to the next hop, encapsulated in an Ethernet or WiFi frame.

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