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How does ARP work with DHCP?

Published in Networking Protocols 4 mins read

ARP (Address Resolution Protocol) and DHCP (Dynamic Host Configuration Protocol) work together to enable devices to communicate on a network. DHCP assigns IP addresses and other network configuration parameters, while ARP resolves IP addresses to MAC addresses, facilitating communication at the data link layer.

Understanding the Roles of DHCP and ARP

  • DHCP: DHCP automates the process of assigning network parameters. According to the provided reference, DHCP automates the assignment of TCP/IP stack setup parameters, such as the default gateway, subnet mask, and IP addresses, when a host connects to the network. This includes:

    • IP Address
    • Subnet Mask
    • Default Gateway
    • DNS Server Addresses

  • ARP: ARP's primary role is to find the MAC address associated with a given IP address on the local network. As per the reference, the ARP protocol is used to resolve the MAC address of a networked device whose IP address is known.

ARP and DHCP in Action: A Step-by-Step Explanation

Here's how ARP and DHCP interact during the network configuration and communication process:

  1. DHCP Request: When a device joins the network, it broadcasts a DHCP Discover message to find a DHCP server.

  2. DHCP Offer: The DHCP server offers an IP address and other configuration information to the device.

  3. DHCP ACK: The device accepts the offer, and the DHCP server sends an acknowledgement (ACK) confirming the IP address assignment. Now the device has an IP address.

  4. ARP Request (Initial Communication): When the device wants to communicate with another device on the same network, and it knows the destination IP address, it needs the destination device's MAC address.

  5. ARP Broadcast: The sending device broadcasts an ARP request packet on the local network. This packet asks, "Who has IP address [destination IP address]? Tell [sender's MAC address]".

  6. ARP Reply: The device with the matching IP address responds with an ARP reply, providing its MAC address.

  7. Communication Begins: Now the sending device has the destination's MAC address, it can encapsulate the IP packets into Ethernet frames and send them directly to the destination device.

Scenario Example

Let's say:

  • Device A (New Device) is connecting to the network
  • Device B (Existing device) is already on the network.
  1. DHCP for Device A: DHCP assigns an IP address (192.168.1.100) to Device A.
  2. Device A pings Device B: Device A knows Device B's IP address is 192.168.1.10. Device A needs to send an IP packet to Device B, it checks its ARP cache.
  3. ARP Request by Device A: If Device B's MAC address isn't in Device A's ARP cache, Device A broadcasts an ARP request: "Who has 192.168.1.10? Tell 192.168.1.100".
  4. ARP Reply by Device B: Device B responds with its MAC address (e.g., 00:11:22:33:44:55).
  5. Device A's ARP Cache updated: Device A updates its ARP cache, mapping 192.168.1.10 to 00:11:22:33:44:55.
  6. Communication Commences: Device A can now send IP packets to Device B.

Key Takeaways

Protocol Function Interaction with Other Protocol
DHCP Assigns IP addresses and network parameters Provides IP addresses that devices use when making ARP requests to find other devices' MAC addresses
ARP Resolves IP addresses to MAC addresses Used to discover the MAC address of a device after it has obtained an IP address from DHCP

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