A MAC (media access control) address is a hexadecimal unique 48-bit physical identifier burned into a network card (CIC) at a factory, working mainly at the OSI model (Data Link) Layer 2 to move data betwen devices on the same LAN.
Unlike IP addresses, which are variable identifiers (at times change), MAC addresses are permenant identifiers (stay the same for that device).
Note: All device have a unique MAC address, never sharing the same one.
Length: 48 bits (6 bytes), represented as 12 hex digits separated by colons or hyphens (e.g., 00:1A:2B:3C:4D:5E).
[2] A MAC address is divided into 2 different identifiers:
[2]
First 3 bytes (or pairs) (24 bits) - Manufacturer ID/OUI (Organizationally Unique Identifier) — Identifies the manufacturer (e.g., Apple, Intel, Cisco).
E.g., a TP link device is assigned a range of available IDs.
[2]
Last 3 bytes (24 bits) - NIC Specific — Unique device ID/Unique hardware serial number assigned by the manufacturer.
E.g.,
MAC vs. IP address
How switches use MAC Addresses (CAM table) When devices communicate on a LAN, network switches dynamically learn each device's location to route traffic efficiently.
MAC Address Table (CAM Table): A switch records the source MAC address of every incoming Ethernet frame and maps it to the physical port it arrived on.
Forwarding vs. Flooding:
If the destination MAC address is already in the table, the switch sends the data only to that specific port (Unicast).
If the destination MAC is unknown or is a broadcast (FF:FF:FF:FF:FF:FF), the switch forwards (floods) the frame out of every port except the one it came in on.
[2] Any device in a home network will have a MAC address and that's how devices know where to send datas. E.g., 2 devices use that MAC address to send to the right device.
E.g., suppose Device A wants to talk to Device B on the same LAN, it uses ARP (Address Resolution Protocol):
Device A kn ows B’s IP address, but needs its MAC address to build the Ethernet frame.
Device A sends an ARP Request to the broadcast MAC (FF:FF:FF:FF:FF:FF).
Device B responds with its specific MAC address.
Switches store these in a MAC Address Table to route frames directly to the correct physical port.
[2] Fig. 1
[2] E.g., suppose this home network has the following router with its ARP table (Fig. 1), without a device connected yet.
address resolution protocol (ARP): a table that ties MAC addresses for devices to their IP addresses
[2] With no other device in the home network yet, the router only has 1 entry in its ARP table: its own IP address (192.168.0.1) mapped to its MAC address (0A:1A:2A:3B:4B:5B).
[2] Fig. 2
Suppose this laptop joins the network (Fig. 2), which also has its own address as the only entry on its table.
[2]
[2] For the laptop to send a request to the public internet, it must send the request through a default gateway, or the router in the home network, and knows that the router is located at 192.168.0.1, but doesn't know its MAC address, thus being unable to send infos yet.
By sending the request to the home address requesting for the "owner" of the 192.168.0.1. The router responds by providing its MAC address 0A:1A:2A:3B:4B:5B to the laptop while requesting for the MAC address of 192.168.0.2 (the laptop). The laptop responds by providing its own MAC address.
[2] Then, both devices can update their ARP tables
[2] If the laptop disconnects from the network and rejoins, it gets a new IP address. In this case, the same process occur with both device updating their tables, with the router now having a another IP address for the laptop (recall the MAC address stay the same as usual).
[2] If a desktop joins, it'll also only have its own address as the sole entry in its table, initially, before all devices request each other's address, then updating others' IP and MAC addresses.
When to identify a device's IP address
[2] When activating our internet plan, we must register our modem to the internet service provider. I.e., you purchase an internet service provider via the ISP, you then must provide the modem that's tied to the internet plan to the ISP from months prior. And to register and link our account to the device, we must get the modem's MAC address.
Another case of requiring to identify a MAC address is on a public WIFI network, we must register the device to use the internet connection via the MAC address.
Unicast: Sent to one specific device (e.g., 00:1A:2B:3C:4D:5E).
Broadcast: Sent to all devices on the local network (FF:FF:FF:FF:FF:FF — all 48 bits set to 1).
Multicast: Sent to a defined group of devices (starts with 01:00:5E for IPv4 multicast).