Module 13: The ARP Process

MAC and IP Addresses

In an Ethernet LAN, devices rely on two types of addresses to communicate: MAC addresses (physical) and IP addresses (logical). When a host needs to send a message but only knows the IP address of the destination, it must determine the corresponding MAC address through a process called address resolution.

Types of Addresses:

• MAC Address (Physical Address) – Used for direct NIC-to-NIC communication within the same Ethernet network.
• IP Address (Logical Address) – Used to route packets between different networks.

If the destination IP address is on the same local network, the sender can resolve the MAC address of the destination device. If the destination IP address is on a remote network, the MAC address used is that of the default gateway (router interface). Routers examine the destination IPv4 address to determine the next-hop device, encapsulating the IPv4 packet in a new data link layer frame for each hop along the path.

Packet Flow in a Network:

1. Same Network: The source device resolves the MAC address of the destination and sends the frame directly.
2. Remote Network: The frame is sent to the MAC address of the default gateway (router), which determines the next hop.
3. Encapsulation Process:
   • The router de-encapsulates the received Ethernet frame.
   • It examines the destination IP address to determine the best path.
   • It encapsulates the IP packet in a new data link frame before forwarding it.
4. Final Destination: If the next-hop device is the intended recipient, the frame’s destination MAC address matches the Ethernet NIC of the receiving device.

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Broadcast Containment

How Broadcast Works

A message can only be sent to one MAC address at a time. However, when a host does not know the MAC address of a destination, it uses broadcasting to send a message to all hosts within the network. The broadcast MAC address is FFFF.FFFF.FFFF (all ones in binary).

When a host sends a broadcast message, all connected devices in the same network receive and process it. This results in network-wide message distribution, which can lead to excessive broadcast traffic if too many hosts share the same network.

Managing Broadcast Traffic

• A broadcast domain includes all devices connected to the same network segment.
• Switches forward broadcast frames to all connected devices within the same local network.
• If too many hosts share the same broadcast domain, excessive broadcasts can slow down the network.
• Routers help divide the network into multiple broadcast domains, preventing unnecessary broadcast propagation beyond local network boundaries.

Selective Frame Acceptance

On a local network, a NIC accepts a frame only if:

1. The destination MAC address matches its own MAC address.
2. The frame is addressed to the broadcast MAC address (FFFF.FFFF.FFFF).

Most network applications rely on logical IP addressing to identify devices, but the sender must still determine the MAC address before communication occurs.

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Address Resolution Protocol (ARP)

When a host needs to determine the MAC address of a device based on its IPv4 address, it uses the Address Resolution Protocol (ARP). ARP follows a three-step process:

1. Broadcast Request:

   • The sending device generates a frame with a broadcast MAC address (FFFF.FFFF.FFFF) and includes the destination IPv4 address in the message.
   • This frame is sent to all hosts within the local network.

2. MAC Address Response:

   • Each receiving host compares the IPv4 address in the message with its own.
   • The device with the matching IPv4 address replies with its MAC address.

3. Updating the ARP Table:

   • The original sender stores the MAC-IP mapping in an ARP table.
   • Future communications with the same device can now bypass the ARP request step.

IPv6 Address Resolution

For IPv6 networks, a similar process called Neighbor Discovery Protocol (NDP) is used instead of ARP. NDP enables devices to discover each other’s MAC addresses and exchange network information efficiently.

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By understanding MAC and IP address resolution, network administrators can optimize communication within a LAN and manage broadcast traffic effectively.