Address Resolution Protocol (ARP) and Routing Examples

ARP is used to find the MAC address for a given IP address.
It operates between Layer 2 (Data Link) and Layer 3 (Network).
When a device wants to send a packet to an IP address on the same local network but doesn't know the MAC address, it uses ARP.
Consider a scenario where a laptop wants to send game data to another laptop with IP address $133.330.30.1$ .
The source laptop needs the destination's MAC address to create a frame.

The sending laptop broadcasts an ARP request on Layer 2 (MAC address is all Fs).
The request asks: "Who has IP address $133.330.30.1$ ?".
The destination laptop, which also runs ARP, recognizes its IP address in the broadcast.
It responds with an ARP reply containing its MAC address (e.g., ending in $5BColon78$ ).
The sending laptop now has the MAC address and can build a frame containing the IP packet.
The frame is sent to the destination laptop.
The destination laptop's Layer 2 strips the frame and passes the packet to Layer 3.
Layer 3 verifies the destination IP and delivers the data to the application (the game).
Even when devices communicate using Layer 3, Layer 2 is used for local communication. Remote communications might involve multiple Layer 2 frames.

Three networks: Orange, Green, and Pink.
Routers: R1 connects Orange and Green, R2 connects Green and Pink.
Routers operate at Layers 1, 2, and 3.
Devices: Two laptops (D1, D2) in the Orange network, one laptop (D3) in the Pink network.

D1 determines D2 is on the same local network using its subnet mask and D2's IP address.
Router R1 is not needed.
Packet P1 is created with D2's IP address as the destination.
ARP is used to get D2's MAC address.
P1 is encapsulated in a frame with D2's MAC address as the destination.
The frame is sent to D2, which strips the frame and packet, delivering the data.

D2 determines D3 is on a different network.
Packet P2 is created with D3's IP address as the destination.
Frame F2 is created, but since D3 is remote, the destination MAC address is the local router (R1).
ARP is used to get R1's MAC address.
The frame F2 is sent to R1.
R1 strips F2, leaving packet P2.
R1 consults its routing table and determines the next hop for the Pink network is R2.
R1 encapsulates P2 in a new frame F3, addressed to R2's MAC address (obtained via ARP).
F3 is sent to R2.
R2 strips F3, leaving packet P2.
R2 consults its routing table and determines D3 is on the same local network.
R2 uses ARP to get D3's MAC address.
R2 encapsulates P2 in a new frame F4, addressed to D3's MAC address.
F4 is sent to D3.
D3 strips F4 and P2, delivering the data to the application.

Routers move packets between networks by reviewing packets and checking route tables for the next hop.
Packets can be re-encapsulated in different Layer 2 frames multiple times during their journey.
The packet itself usually remains unchanged from source to destination.
Routers understand Physical, Data Link, and IP networking.

Layer 2: Device-to-device communication within the same network using MAC addresses.
Layer 3 Addition:
- IP addresses (v4, v6) for cross-network addressing.
- ARP to find MAC addresses for given IP addresses.
- Routes: Define where to forward packets.
- Route Tables: Contain multiple routes.
- Routers: Move packets between networks, encapsulating them in Layer 2 frames.
- Device-to-device communication over the internet.

No method of individual communication channels between two devices. Only supports single stream of communication.
Packets can potentially be delivered out of order due to varying network conditions.