---
title: "Short note: ARP"
subject: "Computer Networks and Security"
chapter: 4
key: "semester_7/computer_networks_and_security:arp-note"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/arp-note
site: BCT Engineering
---

# Short note: ARP

Computer Networks and Security · Chapter 4: Network Layer

- Asked in: 2071 Shrawan, 4 marks
- Video: [Address Resolution Protocol (ARP) Explained](https://www.youtube.com/watch?v=xTOyZ6TWQdM) — Networklessons.com
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/arp-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-4

## Question

Write a short note on ARP.

## Short answer

ARP (RFC 826) resolves a known IP address to its MAC address on a LAN. It broadcasts an ARP request, the owner sends a unicast reply, and the mapping is cached; for remote hosts, ARP asks for the default gateway's MAC.

## Answer

**ARP (Address Resolution Protocol, RFC 826)** finds the MAC (data-link) address of a host when only its IP address is known. It is needed because frames are delivered using MAC addresses on a LAN.

#### Operation
1. A host wants to send to IP *B* on the same network. It checks its **ARP cache**.
2. If no entry exists, it broadcasts an **ARP request** (destination MAC FF:FF:FF:FF:FF:FF): "Who has IP B? Tell A."
3. All hosts receive it, but only B replies with a unicast **ARP reply** containing its MAC.
4. A stores the mapping in its cache (for a few minutes) and sends the frame.

If the destination is on another network, the host ARPs for the **default gateway's** MAC instead.

```text
A: "Who has 192.168.1.5?"  --broadcast-->  all
B: "192.168.1.5 is at 00:1A:2B:3C:4D:5E" --unicast--> A
```

#### ARP packet fields
Hardware type, protocol type, hardware length (6), protocol length (4), operation (1 request, 2 reply), sender MAC, sender IP, target MAC, target IP (28 bytes for Ethernet/IPv4).

Related: **RARP** (MAC to IP, obsolete), **Proxy ARP** (a router answers for hosts on another network), **Gratuitous ARP** (announces own mapping).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does ARP find?
   A. The port of a service
   B. The MAC address of a host when its IP address is known
   C. The route to a remote network
   D. The IP address from a known domain name
   Answer: B. ARP maps a known IP address to the MAC address needed for frame delivery on the LAN. (Key point: Purpose of ARP)

2. How are the ARP request and reply sent?
   A. Both are unicast
   B. Request is unicast, reply is broadcast
   C. Request is broadcast to FF:FF:FF:FF:FF:FF, reply is unicast from the target
   D. Both are broadcast
   Answer: C. The request goes to all hosts; only the owner of the IP replies directly to the asker. (Key point: ARP request/reply)

3. If the destination IP is on a different network, whose MAC does the host ARP for?
   A. The nearest switch
   B. The default gateway
   C. The DNS server
   D. The destination host
   Answer: B. The frame must first reach the default gateway (router), so its MAC is resolved. (Key point: Destination on another network)

4. Which variant is a router answering ARP requests on behalf of hosts on another network?
   A. RARP
   B. Proxy ARP
   C. ARP cache
   D. Gratuitous ARP
   Answer: B. Proxy ARP lets a router reply for hosts that are on another network. (Key point: ARP variants)