ARP (Address Resolution Protocol)

Why do we need ARP?

We know that every device has an IP address.

Example:

Laptop
IP: 192.168.1.10

Router
IP: 192.168.1.1

But computers don't send Ethernet frames using IP addresses.

On a local network (LAN), devices communicate using MAC addresses.

So a computer often knows:

Destination IP:
192.168.1.20

But it doesn't know:

Destination MAC:
??

That's the problem ARP solves.

ARP maps an IP address to its corresponding MAC address on a local network.

What is a MAC Address?

A MAC (Media Access Control) address is a unique hardware identifier assigned to a network interface card (NIC).

Example:

IP Address:
192.168.1.20

MAC Address:
00:1A:2B:3C:4D:5E

Think of it like:

IP Address → House address (can change depending on the network)
MAC Address → Person's unique identity card for that network interface

Why isn't the IP address enough?

Suppose your laptop wants to send data.

Laptop

↓

Destination IP:
192.168.1.20

When the packet reaches the Data Link Layer (Ethernet), it must be placed inside an Ethernet frame.

That frame requires:

Destination MAC

Without the MAC address, the frame cannot be delivered on the local network.

ARP Request

Suppose:

Laptop

IP:
192.168.1.10

MAC:
AA-AA-AA-AA-AA-AA

Needs to send data to:

192.168.1.20

It checks its ARP cache.

192.168.1.20

↓

Not Found

So it broadcasts an ARP request.

ARP Request

The laptop sends:

Who has 192.168.1.20?

Tell 192.168.1.10

This is sent as a broadcast.

Broadcast MAC:

FF:FF:FF:FF:FF:FF

Meaning:

Every device on this local network, please listen.

Every Device Receives It

Imagine the LAN:

Laptop

↓

Broadcast

↓

PC1

PC2

Printer

Router

Phone

Every device receives the request.

Each device checks:

Is my IP 192.168.1.20?

Most devices answer:

No.

One device answers:

Yes.

ARP Reply

The matching device responds:

192.168.1.20

is

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

Unlike the request, this reply is usually unicast (sent only to the requester).

ARP Cache

The laptop stores the mapping.

ARP Cache

192.168.1.20

↓

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

Now future packets don't need another ARP request until the cache entry expires.

Complete ARP Flow

Laptop wants to send data
        │
        ▼
Knows destination IP
        │
        ▼
Checks ARP Cache
        │
        ├── Found
        │      │
        │      ▼
        │  Send Ethernet Frame
        │
        ▼
Not Found
        │
        ▼
Broadcast ARP Request
        │
        ▼
Destination replies with MAC
        │
        ▼
Store in ARP Cache
        │
        ▼
Send Data

Example

Suppose:

Laptop

IP:
192.168.1.10

Wants to reach:

Web Server

IP:
192.168.1.20

Step 1:

Need MAC for

192.168.1.20

Step 2:

Broadcast

Who has 192.168.1.20?

Step 3:

Server replies

My MAC is

00:AB:CD:EF:12:34

Step 4:

Laptop stores

192.168.1.20

↓

00:AB:CD:EF:12:34

Step 5:

Data transmission begins.

Where Does ARP Work?

ARP works only on the local network (LAN).

It cannot discover the MAC address of a device on another network.

Example:

Your Laptop

↓

192.168.1.x

↓

Internet

↓

Google Server

Your laptop does not use ARP to find Google's MAC address.

Instead, it uses ARP to find the MAC address of its default gateway (router).

The router then forwards the packet towards its destination.

ARP vs DNS

Many beginners confuse these.

DNS	ARP
Domain → IP	IP → MAC
Works across networks	Works only on the local network
Example: `google.com` → `142.x.x.x`	Example: `192.168.1.20` → `00:1A:2B:3C:4D:5E`

--- # ARP Cache Operating systems maintain an ARP cache to avoid broadcasting for every packet.

Example:

IP Address          MAC Address

192.168.1.1         11:22:33:44:55:66

192.168.1.20        00:1A:2B:3C:4D:5E

Entries expire after a period so that changes on the network can be discovered.

Interview Questions

Why do we need ARP?

Because Ethernet delivers frames using MAC addresses, while applications and IP routing use IP addresses.

ARP translates an IP address into a MAC address on the local network.

Is ARP used on the internet?

No.

ARP is used only within a local network.

Across the internet, routers forward packets using IP routing.

Each router may use ARP on its own local network to communicate with the next hop.

Is ARP a broadcast protocol?

ARP Request: Yes, it is broadcast.
ARP Reply: Usually unicast.

Does ARP use TCP or UDP?

Neither.

ARP is its own protocol that operates at the boundary of the Data Link Layer (Layer 2) and Network Layer (Layer 3).

It does not use TCP or UDP.

Notion Notes (Short Version)