ARP

Address Resolution Protocol (ARP) maps IP address to MAC addresses at Layer 2
ARP requests and replies are Frames that include the target IP address, the sender's MAC Address, and either the Broadcast MAC address or the target's MAC Address
- If the Switch doesn't have the know the target MAC address, it broadcasts out the request to all ports
- Hosts that receive frames with mismatched target MAC addresses will drop the frame
- The source and destination IP addresses never change
- The MAC address changes at each hop
- At each hop, the ARP cache is updated to expedite future requests.
Mapped addresses are saved to a host's ARP Cache and the Switch's MAC Address Table, which allows the host to send unicast Unicast packets directly to the destination
When sending an ARP request outside of the subnet, the host will first contact its Default Gateway
- Sender wants to send ARP request to destination outside of its subnet
- It first sends an ARP request to its Default Gateway's IP address
  - the DG replies with its own MAC address
- The sender then sends a request with the Target's IP, but uses the DG's MAC address
- The DG then forwards the ARP request to the appropriate subnet
  - The target replies with its MAC address
- The DG then forwards to reply to the sender

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