ddarwactor
07-24-2011, 12:37 PM
Hi, all! Newbie, here, and this'll be my first post so please forgive me if I'm not in the right place to post this. I'm new to forums in general and presume this is the place to "think out loud". Please, correct me if I'm wrong, thanks!
So, I'm reading along at pg 99 (E-reader version, 7th ed.) about ARP and am wondering why it's necessary for IP to "broadcast" looking for a device with a known IP address but an unknown (not in the ARP cache) hardware address. If the IP address is known, why can't the query for that device's hardware address be directed solely to that IP, rather than forcing all devices on that segment to listen to the query? Isn't the idea to keep broadcasts to a minimum?
If the device has an IP address, for that matter, how did it get it without having it's hardware address placed in the ARP cache in the process?
Any clarification would be greatly appreciated and thanks for taking the time to read this! :)
Edit: I think I may have answered my own question - someone please correct me if I'm wrong. It appears that the ARP broadcast originates from a device that needs but doesn't yet have in its cache the MAC address of another device that it wants to communicate with (say, PC to PC)...and both values (logical and physical address) are needed before direct comms can proceed between them. Although, seems to me that the destination's MAC address would already reside in either a router or switch's ARP cache - but, there's no "ARP server" whose job it is to answer requests for MAC info from any requesting device. So, the ARP broadcast may well pass through devices that already know both the IP and MAC that the source is asking for but it's not their job to answer the query - hence the necessity for a broadcast? I think I'm talking myself in circles, here...
So, I'm reading along at pg 99 (E-reader version, 7th ed.) about ARP and am wondering why it's necessary for IP to "broadcast" looking for a device with a known IP address but an unknown (not in the ARP cache) hardware address. If the IP address is known, why can't the query for that device's hardware address be directed solely to that IP, rather than forcing all devices on that segment to listen to the query? Isn't the idea to keep broadcasts to a minimum?
If the device has an IP address, for that matter, how did it get it without having it's hardware address placed in the ARP cache in the process?
Any clarification would be greatly appreciated and thanks for taking the time to read this! :)
Edit: I think I may have answered my own question - someone please correct me if I'm wrong. It appears that the ARP broadcast originates from a device that needs but doesn't yet have in its cache the MAC address of another device that it wants to communicate with (say, PC to PC)...and both values (logical and physical address) are needed before direct comms can proceed between them. Although, seems to me that the destination's MAC address would already reside in either a router or switch's ARP cache - but, there's no "ARP server" whose job it is to answer requests for MAC info from any requesting device. So, the ARP broadcast may well pass through devices that already know both the IP and MAC that the source is asking for but it's not their job to answer the query - hence the necessity for a broadcast? I think I'm talking myself in circles, here...