ARP (Address Resolution Protocol)

🤔 What is ARP? (Basic Idea)

In a network:

• Devices communicate using IP addresses (logical)

• But actual data delivery happens using MAC addresses (physical)

📌 Problem:

How does a device find the MAC address of another device when it only knows the IP address?

👉 Answer: ARP

🧠 Simple Definition

📌 ARP (Address Resolution Protocol) is a protocol used to map an IP address to its corresponding MAC address within a local network.

🧠 Why ARP is Needed?

Let’s understand the mismatch:

📌 IP is used for routing,

📌 MAC is used for actual delivery.

👉 ARP acts as a bridge between these two layers.

🧠 Real-Life Analogy

🏠 House Address vs Door Number

• You know someone’s house address (IP)

• But to deliver a parcel, you need the door number (MAC)

👉 ARP finds the door number for you 📦

🧭 Where Does ARP Work?

• ARP works between:

  • Network Layer

  • Data Link Layer

📌 Often called a cross-layer protocol.

🔢 What Addresses Does ARP Use?

• Logical Address → IP Address

• Physical Address → MAC Address

Example:

IPAddress:192.168.1.10
MACAddress:00:1A:2B:3C:4D:5E

🧾 ARP Table

📌 Every device maintains an ARP table (ARP cache).

🧠 What is ARP Table?

• Stores IP ↔ MAC mappings

• Speeds up communication

• Avoids repeated ARP requests

🧾 Example ARP Table

📌 Entries have a timeout (TTL).

🔄 How ARP Works (Step-by-Step)

Let’s say:

• Device A wants to send data to Device B

• Device A knows IP of B, not MAC

🪜 ARP Working Steps

1️⃣ Device A checks its ARP table

2️⃣ MAC not found ❌

3️⃣ Device A sends ARP Request (Broadcast)

4️⃣ All devices receive request

5️⃣ Device B replies with ARP Reply (Unicast)

6️⃣ Device A stores MAC in ARP table

7️⃣ Data transmission starts

📌 Simple and fast ⚡

📡 ARP Request vs ARP Reply

📦 ARP Packet Format (Basic Idea)

ARP packet contains:

• Sender IP

• Sender MAC

• Target IP

• Target MAC

📌 Used to exchange address information.

🧠 Types of ARP

⚠️ ARP Security Issues (ARP Spoofing)

🧠 What is ARP Spoofing?

• Attacker sends fake ARP replies

• Victim updates ARP table wrongly

• Attacker becomes man-in-the-middle

📌 Very dangerous attack 🚨

🛡️ Prevention

• Static ARP entries

• ARP inspection

• Switch security features

🧠 ARP vs RARP (Quick Note)

📌 RARP is mostly obsolete.

🧠 Memory Tricks

• ARP = Ask for MAC

• Request = Broadcast 📢

• Reply = Unicast 📩

• IP → MAC

⭐ One-Line Summary

🔹 ARP is a protocol that resolves an IP address into a MAC address to enable data delivery within a local network.