All devices communicate locally with each other using their mac addresses. ARP is used to allow mapping the IP address which to upper layer protocols know about to the mac address for the devices to communicate with.
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Why ARP
Hello!
I cant get my overworked head around ARP (address resolution protocol) I know it translates ip addresses to MAC addresses but I dont understand why for example my pc my PC would need to know the MAC address of a router as well as its internet address. I know I am beig dumb here but its late and I need noodles :(
Thanks for any clear explainations :)
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Answers
Packets ultimately get sent out at layer 1 - physical signalling - voltage pulses or light signals on fiber etc.. Before we signal out over layer 1 the device needs to know what to signal. So at layer 2 is where we use MAC addresses. Every packet that goes out from your PC is a layer 2 packet or more correctly called a datagram I believe. So the layer 2 packet contains the source MAC address (i.e. you NIC's mac address) and the destiatnion MAC address (i.e. the mac address of your default gateway of or of the device on the same network that you are talking to. So until your machine knows the layer 2 MAC address of where the packet is going - it doesn't know who to send it to.
Inside the layer 2 datagram is the data portion which is the layer 3 packet. The layer 3 packet contains the source and destination IP address. Again the source is your IP address and the destination is the actual destination IP address to which you are tyring to send your packet.
So the construction of the layer 3 packet is easy.. Source and destination ip address and the data and some other control stuff. Now the PC or any network device has to figure out how to get the layer 3 packet into the correct layer 2 datagram. So at this point it knows your MAC address but it may not know the MAC address of the device you are sending it to. At layer 2 you will be sending it to a device on your local network which in many cases will be your default gatway.
Before we get to the next point it is important to realize the decision process your PC or network device will make before it attempts to create the layer 2 datagram:
1) Is the destination IP address on my IP network (same subnet as the sending device)?
If yes - then we need to send the packet directly to that device and therefore I need the MAC address of that specific device
If no - then that device lives on another network and therefore I need the MAC address of my default gatway. (BTW.. I know the IP address of my default gateway because that is already configured.)
During the construction of the layer 2 datagram we will need either the MAC address of the specific device (if its on our same subnet) or the MAC address of the default gateway. So the network device looks and sees if the IP address of the specific device or default gateway at exists in the current ARP table.
If it is there then we have the MAC address that relates to that IP address and we can build and send the layer 2 packet.
If not then we have to go through the ARP process where we are simply sending out a broadcast packet asking if anyone has this IP address - to please send me back your MAC address.
Once we get the response we stick it in the ARP table and complete the building of the layer 2 datagram and then we can send the packet out.
Hope that makes sense.
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by: MikeHolcombPosted on 2009-10-28 at 10:06:28ID: 25685526
When two systems located on the same LAN want to communicate, they do not transfer data to and from systems based on IP address but rather than the MAC address. Unlike an IP address that can be assigned so duplicates exist, no two network interfaces "should" have the same MAC address ensuring each system is uniquely represented.
IP comes into play when systems located on different networks need to communicate, allow that communication to be routed to the remote destination through a series of routers based on destination IP.
I would review http://en.wikipedia.org/wi
Hope this helps...
Mike