Introduction to TCP/IP and routing : Introduction to IP : IP on different medias ("links") : IP addressing <-> link level addressing
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1.4.1. IP addressing <-> link level addressing

Networks like Token Ring and Ethernet have their own addressing mechanisms on 48 bits, represented as 6 hexadecimal bytes:

	AA:BB:CC:DD:EE:FF

On local networks, IP hosts communicate using IP, but need a way to find each other's physical address.

In the case of Ethernet and Token Ring (also FDDI), and other IEEE 802 networks, this mechanism is called ARP (Address Resolution Protocol, RFC 826).

The abovementioned protocols support broadcast, which means that it is possible to send a packet to the entire network, where every host can receive it.

Using this facility, two hosts can find each other by broadcasting requests for each other's MAC/LLC address, as follows:

host A (IP address = 1.0.0.8, MAC address = 00:01:02:03:04:10) host B (IP address = 1.0.0.45, MAC address = 00:01:02:03:04:15)

  1. host A: broadcast request: "who has 1.0.0.45, tell 1.0.0.8"
  2. host B: broadcast reply: "1.0.0.45 is at 00:01:02:03:04:15"

Seen through the UNIX tools tcpdump(8):

  12:21:30.275710 arp who-has 1.0.0.45 tell 1.0.0.8
  12:21:30.275817 arp reply 1.0.0.45 is-at 0:1:2:3:4:15

The response is then cached for a limited time for reuse, so that a new request need not be made for each transmission.


Introduction to TCP/IP and routing : Introduction to IP : IP on different medias ("links") : IP addressing <-> link level addressing
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Next: IP and point-to-point links