Networking Basics · 6

🔀 Switching & MAC Addresses

How data finds the right device inside your network⏱ ~2 min

🏢The smart office receptionist

Imagine an office where a receptionist sits at the front desk. The first time someone asks for 'Alice in Accounting,' the receptionist announces it over the intercom. But after Alice walks past, the receptionist remembers: 'Alice is at desk 14.' Next time, they send the visitor directly there — no intercom needed. A network switch works exactly the same way, learning which device is on which port and delivering frames directly.

MAC Addresses — the hardware fingerprint

  • •Every network card (wired or wireless) has a MAC address assigned by the manufacturer — stored in firmware, not truly 'burned in' (it can be changed in software, which is called MAC spoofing)
  • •Format: 6 pairs of hex digits — AA:BB:CC:DD:EE:FF
  • •First 3 bytes (OUI) = manufacturer. Last 3 bytes = unique device ID
  • •AA:BB:CC = made by, say, Intel. DD:EE:FF = the specific card number
  • •Unlike IP addresses, MACs don't change when you move networks (usually)

How a Switch Learns (MAC Table)

  1. 1.Device A sends a frame from port 1 → switch records 'A's MAC = port 1'
  2. 2.Switch looks up destination MAC in its table
  3. 3.Known destination → delivers directly to that port only
  4. 4.Unknown destination → floods all ports (except where it came from)
  5. 5.Over time, switch builds a complete table — no more flooding needed
Hub (old, avoid!)
  • •Sends every packet to EVERY port
  • •Every device sees all traffic
  • •A hacker on one port sees everything
  • •Creates massive collision domains
  • •Almost never used today
Switch (modern)
  • •Sends frames only to destination port
  • •Devices only see their own traffic
  • •Much harder for attackers to sniff
  • •Separate collision domain per port
  • •The standard in all modern networks

VLANs — virtual separation on one physical switch

  • •VLAN 10 = Students, VLAN 20 = Teachers — logically separated even on the same switch
  • •Traffic between VLANs must go through a router (inter-VLAN routing)
  • •A student on VLAN 10 cannot reach a server on VLAN 20 without a router allowing it
  • •Huge security tool — isolates printers, cameras, guest WiFi, critical servers
🔒 SecurityMAC Flooding Attack: An attacker floods the switch with thousands of fake MAC addresses, filling up the MAC table. When the table is full, the switch panics and starts acting like a hub — sending every frame to every port. Now the attacker can capture all traffic. Defense: enable port security to limit MACs per port.
🧠Quick Checkfirst try = +5 XP

A switch receives a frame destined for MAC address FF:FF:FF:FF:FF:FF. What does it do?

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