Address Resolution Protocol in Networking | Generated by AI
Question: What is ARP in networking?
Answer: ARP (Address Resolution Protocol) is a fundamental protocol in TCP/IP networks that translates IP addresses (Layer 3) into MAC addresses (Layer 2), allowing devices on the same local network (LAN) to communicate at the data-link layer.
How ARP Works
- A device (e.g., Computer A) wants to send data to another device (Computer B) on the same LAN and knows Computer B’s IP address but not its MAC address.
- Computer A broadcasts an ARP Request packet to the entire local network:
“Who has IP address 192.168.1.10? Tell 192.168.1.5” (including its own MAC and IP). - All devices on the LAN receive the broadcast, but only the device with the matching IP (Computer B) replies with an ARP Reply (unicast):
“IP 192.168.1.10 is at MAC 00:1A:2B:3C:4D:5E”. - Computer A receives the reply, updates its local ARP cache (a temporary table of IP-to-MAC mappings), and can now send Ethernet frames directly to Computer B’s MAC address.
Key Points
- Operates at Layer 2 (Data Link) of the OSI model but is required for Layer 3 (IP) communication on LANs.
- Used only within the same broadcast domain (same subnet/LAN). Routers do not forward ARP broadcasts.
- ARP is stateless and connectionless.
- Entries in the ARP cache typically expire after a few minutes (e.g., 2–10 minutes depending on OS) to handle changes in hardware.
ARP Packet Structure (simplified)
| Field | Purpose |
|---|---|
| Hardware Type | Usually 1 (Ethernet) |
| Protocol Type | Usually 0x0800 (IPv4) |
| Hardware Address Length | 6 bytes for MAC |
| Protocol Address Length | 4 bytes for IPv4 |
| Operation | 1 = Request, 2 = Reply |
| Sender MAC & IP | Source device info |
| Target MAC & IP | Destination info (target MAC is 00:00:00:00:00:00 in requests) |
Variants and Related Protocols
- Gratuitous ARP: A device announces its own IP-to-MAC mapping (used when IP changes or to detect duplicates).
- Proxy ARP: A router answers ARP requests on behalf of devices in another subnet.
- ARP Spoofing/Poisoning: A common attack where a malicious device sends fake ARP replies to associate its MAC with someone else’s IP (basis for man-in-the-middle attacks).
Example (Linux/Windows command)
# View ARP table
arp -a # Windows
ip neigh show # Linux
In short: Without ARP, devices on a local network wouldn’t know which physical (MAC) address belongs to a given IP address, and Ethernet communication would fail.