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How Are ARP Tables Built?

Published in Network Protocols 3 mins read

An ARP (Address Resolution Protocol) table is built by collecting responses to ARP queries. These queries and responses are logged before a device transmits a packet on the network. Let's break down the process:

Understanding ARP and its Table

The Address Resolution Protocol (ARP) is crucial for network communication. It translates an IP address (like 192.168.1.100) into a MAC address (like 00:1A:2B:3C:4D:5E), which is necessary for devices on the same local network to communicate directly. The ARP table acts as a cache, storing these IP-to-MAC address mappings for efficiency.

The ARP Table Building Process

  1. ARP Request: When a device needs to send a packet to another device with a known IP address but an unknown MAC address, it broadcasts an ARP request on the local network. This request asks: "Who has this IP address? Tell me your MAC address."

  2. ARP Reply: The device with the target IP address responds with an ARP reply. This reply contains its MAC address.

  3. Table Update: The requesting device receives the ARP reply and adds an entry to its ARP table. This entry includes the IP address and the corresponding MAC address.

  4. Caching: Subsequent communications to the same IP address can use the cached MAC address from the ARP table, avoiding the need for further ARP requests. This significantly speeds up network communication.

Example: Imagine your computer wants to send data to a printer with the IP address 192.168.1.101. Your computer doesn't know the printer's MAC address. It broadcasts an ARP request. The printer responds with its MAC address. Your computer adds an entry (192.168.1.101, Printer's MAC Address) to its ARP table.

Key Aspects to Note

  • Dynamic Nature: ARP tables are dynamic. Entries are added when ARP replies are received and may be removed after a certain period of inactivity (time-to-live).
  • Local Scope: ARP tables only contain entries for devices on the same local network segment.
  • Manual Updates: While not common, ARP tables can sometimes be manually updated using commands like arp (Linux/macOS) or arp -s (Windows). Tools like arping can facilitate this process. However, relying on automatic updates through ARP requests is the standard practice.
  • Security Implications: ARP table entries can be manipulated through attacks like ARP poisoning. This is why secure networks often employ ARP inspection mechanisms.

The information provided aligns with statements from multiple sources, including the provided snippets, which confirm that the ARP table is constructed by logging the responses to ARP queries before packet transmission on the network.

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