Ethernet, MAC addresses and switches

Inside a home, office or university building, computers are joined by Ethernet cables or Wi-Fi into a local area network (LAN). The link layer moves frames from one device to the next one on the same network.

MAC addresses

Every network card has a 48-bit MAC address, written as six hex pairs such as 3c:22:fb:91:0a:7e. It is set by the maker and names the card, not its location. ff:ff:ff:ff:ff:ff is the broadcast address: a frame sent to it reaches every device on the LAN.

An Ethernet frame holds the destination MAC, the source MAC, a type (for example "IPv4 inside"), the data (46 to 1500 bytes) and a CRC-32 (see the previous lesson).

Switches learn by themselves

A switch joins many devices. It keeps a table: MAC address → port. Nobody fills this table in. For every frame that arrives:

1. Learn: the frame's source address is reachable through the port it came in on.
2. Forward: if the destination is in the table, send the frame out of that one port.
3. Flood: if not (or it is a broadcast), send it out of every port except the one it came from.

table = {}                       # MAC address -> port
frames = [("A", "B", 1), ("B", "A", 2), ("C", "A", 3), ("A", "C", 1), ("B", "ff", 2)]

for src, dst, port in frames:
    table[src] = port            # learn where the sender is
    if dst in table and dst != "ff":
        print(src, "->", dst, ": forward out of port", table[dst])
    else:
        print(src, "->", dst, ": flood out of every port but", port)
print("table:", table)

After a few frames, almost nothing is flooded. An old hub repeated every frame to every port, so all devices shared one link and their frames could collide; a switch gives each port its own.

ARP: from an IP address to a MAC address

Programs use IP addresses, but a frame needs a MAC address. ARP (Address Resolution Protocol) asks the whole LAN: "Who has 192.168.1.1? Tell 192.168.1.10." The owner answers with its MAC address, and the asker keeps the answer in its ARP cache for a few minutes.

lan = {"192.168.1.1": "aa:aa:aa:00:00:01", "192.168.1.20": "aa:aa:aa:00:00:20"}
cache = {}

def mac_for(ip):
    if ip in cache:
        return cache[ip] + " (from the cache)"
    print("broadcast: who has", ip, "?")
    if ip not in lan:
        return "no answer"
    cache[ip] = lan[ip]          # the owner replies; remember it
    return cache[ip]

print(mac_for("192.168.1.1"))
print(mac_for("192.168.1.1"))
print(mac_for("192.168.1.99"))

To reach a computer on another network, your computer does not ARP for it: it sends the frame to the MAC address of its default gateway (the router), which passes the packet on.

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