beginner v1 • Updated 7 hours ago

ARP (Address Resolution Protocol)

ARP is a Layer 2/3 protocol that resolves known Layer 3 logical IP addresses into Layer 2 physical MAC addresses on a local network.

Remember This Key Rule

ARP maps known IP -> unknown MAC. ARP Request = Broadcast (FF:FF:FF:FF:FF:FF); ARP Reply = Unicast.

In Simple Terms

When Host A wants to send an IP packet to Host B on the same LAN, it knows Host B's IP address, but its network card cannot build an Ethernet frame without Host B's destination MAC address. ARP asks everyone on the local network: 'Who has IP 192.168.1.20? Tell 192.168.1.10!' Host B replies with its MAC address.

Visual Diagram & Flow

Host A (192.168.1.10)

1. ARP Request (Broadcast)
──►
◄──
2. ARP Reply (Unicast)

Host B (192.168.1.20)

Why It Matters in Real Networks

Without ARP, local IPv4 communication over Ethernet would be completely impossible.

How It Works (Technical Breakdown)

1. **ARP Request**: Sent as a Layer 2 Broadcast (`FF:FF:FF:FF:FF:FF`). Every device in the broadcast domain receives it. 2. **ARP Inspection**: Devices check if their IP matches the requested target IP. 3. **ARP Reply**: The target device unicasts its MAC address directly back to the requester. 4. **ARP Cache (Table)**: Both devices cache the IP-to-MAC mapping in RAM for several minutes to avoid sending repeated broadcasts.

Real-World Analogy

Standing in a conference room and shouting: "Who is Alice Smith? Please tell Bob!" Alice hears her name, raises her hand, and hands Bob her business card.

Concrete Example

PC1 runs `arp -a` in Windows cmd to view cached IP-to-MAC pairs: `192.168.1.1 00-14-22-01-23-45 dynamic` `192.168.1.25 a4-83-e7-11-22-33 dynamic`

Common Engineering Mistakes & Pitfalls

Thinking ARP requests travel across routers. ARP is strictly a Layer 2 broadcast that stops at the router boundary.

Prerequisites

MAC Address & Ethernet Frame Structure Recommended IPv4 Addressing & Classless Architecture (CIDR) Recommended

Next & Related Concepts

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Concept Mastery Quiz

Test your comprehension with active-recall questions.

1. What is the destination Layer 2 MAC address used by an ARP Request frame?

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Content Versioning

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Version	Change Summary	Author	Timestamp
v1 (Current)	Active production content	system	Sun, Aug 23, 2026 11:09 AM