Internet Video Series, Part 2
Episode 18, Segment 05 of 06
When your computer needs to know the Ethernet address of another computer
whose IP address indicates that it should reside within your local network, it
broadcasts an ethernet message that looks like this:
"Hey! Everybody Listen Up! According to my analysis, the IP address 192.168.0.2 should belong to somebody on this Ethernet LAN, and I have an IP message for that destination. If - YOU - are the station that has been assigned that IP address, please respond back to me on - my - Ethernet address so - I - can collect - your - Ethernet address."
If the destination address is incorrect, unassigned, or if the destination computer is switched off or disconnected, then no response message will be received, and your computer will abandon the attempt to send the message.
Under normal circumstances on a healthy network, within a few milliseconds, an answering message will be received like this:
(Click small image above to see an expanded version)
That message provides all of the information your computer needs, and it will construct an Ethernet frame directed at Ethernet Address 31, 22, 3D, 01, 01, A2, containing the IP packet needing transmission. Your local computer's operating system will automatically store a note within a special, temporary memory area known as the - "ARP cache", - recording the received IP and Ethernet address relationship for future reference. Typically, the ARP cache information is held for only a few minutes (so it is handy for long enough to complete current tasks) before it is automatically cleared out.