ARP, Layer 2 Broadcasts, Switches and NICs
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robroy
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ARP, Layer 2 Broadcasts, Switches and NICs
Hardware, Networking, Switches, Hubs, Routers, Cabling
How does ARP work with layer 2 broadcasts? What I mean is, if an IP address is not yet in its cache, ARP broadcasts an ARP request in order to discover the corresponding MAC address, right? Does that mean that the hardware address of the Ethernet frame that encapsulates the ARP request is ff:ff:ff:ff:ff:ff?

Also, sometimes the upper layers request IP broadcasts ? i.e. ARP must sometimes process packets whose destination IP address is a broadcast address, e.g. 192.168.1.255/24, right? So does it automatically map this to ff.ff.ff.ff.ff.ff? I.e. is this built into the ARP protocol?

When the switch receives the frame with the destination address of ff:ff:ff:ff:ff:ff, it sends it out of all its ports, except the port that the frame came in on, right? I guess this means that there is logic built into the switch?s circuitry that recognises frames with the destination address of ff:ff:ff:ff:ff:ff, right?

Finally, each NIC in that network receives and processes the broadcast frame, right? So I guess this also means that there is logic built into the NIC?s circuitry that recognises frames with the destination address of ff:ff:ff:ff:ff:ff, right?
ASKED: Nov 25 2005  9:11 AM GMT
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What exactly are you trying to achieve? Are you having a conflict with some addresses, or are you monitoring network traffic and bottlenecks?
Or you are trying to understand how ARP works and RARP?

Regards

Newton
Last Answered: Dec 2 2005  0:50 AM GMT by Guardian   0 pts.
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boardinhank   60 pts.  |   Dec 6 2005  8:38AM GMT

Not sure what you are looking for but here is what arp does. ARP is used to map IP network addresses to the hardware addresses used by a data link protocol, meaning you ping 192.168.0.1 your NIC sends a broadcast to the Switch and askes who has this IP address. It is broadcast to all ports. once the target system is contacted and replys then the switch makes note of the mac address on that port for future traffic to that mac. Once you receive the reply you add that IP addresses MAC to your mac table. Now when you try to communicate with that IP again the MAC you have in your table will be used in the packet as well. the switch reads that mac in your traffic and references its mac table and forwards it to where it needs to go.

As for your nic, your nic is using a protocol to communicate. like you and I use english your nic uses tcp/ip in this case. the nics logic is it knows how to speak tcp/ip and tcp/ip has the communication ability of ip addresses and mac addresses to start and carry on network communication. Play with it a bit. Run arp -a and make not of what it listed. Try to ping an IP address in your local subnet that you do not see in that list. if it responds run arp -a again. you will see its entry and mac adddress in there. the MAC is a uniqe identifier that in most cases will never be duplicated, at least not in your own region.. :) thats why it is used to communicate over your LAN, cuts down on confusion and configuration issues.

 
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