Week 3 - samuelwilson02/tech-journal GitHub Wiki

LANs and Layer 2

• in a LAN: computes communicate NIC to NIC, sending packets w/ their own MAC as destination
• they pay attention to Layer 2 broadcasts (FFs in destination MAC

Layer 2 = data link layer

Hubs (Layer 1, physical) (obsolete)

• don't care about destination - just broadcast packets out all ports
• NICs either accept or reject packets based on MAC destination -> inefficient

Switches (Layer 2 - used in most modern LANs)

• direct packets correctly instead of sending to everyone
• Store info about devices to determine where to send packets -> MAC/CAM/Lookup table
• Correlates devices to switch ports
• If the lookup table doesn't recognize a MAC, it will "flood" the ports
• Filter or flood: recognized -> send to right port (not broadcast); unrecognized -> flood
• Full duplex: can send/receive in both directions

ARP (Address Resolution Protocol)

• Systems find each other's MAC addresses with ARP
• A device must know the destination MAC for its NIC to accept a packet
• When MAC = all "F"s any NIC will accept it (forwarding device)
• ARP identifies the destination IP, requests the MAC; device with the matching IP sends back its MAC
• OS can use an ARP cache to store MACs of recently resolved IP addresses
• MACs are only relevant within the LAN - outside the LAN, ARP is irrelevant because the next MAC is the default gateway
• The Layer 2 header (MAC) is rewritten as it crosses each router; IP (layer 3) stays the same
• If the destination IP's network ID doesn't match the source's, it sends to the default gateway
• On larger networks, ARP can cause issues - so networks are separated into subnets connected by routers