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Intermediate Topic 27 of 50 Computer Networks

ARP

Protocol mapping IP addresses to MAC addresses.

How it works

The Address Resolution Protocol maps an IPv4 address to a MAC address on a local network. A host that wants to reach an IP on its subnet broadcasts an ARP request asking who has this address, and the owner replies with its MAC address. Results are cached in an ARP table for a short time. Gratuitous ARP announces a change of address or detects duplicates. IPv6 uses Neighbor Discovery instead.

Step-by-step walkthrough

  1. A wants to send to 192.168.1.9 but needs the MAC.
  2. It broadcasts who has 192.168.1.9.
  3. Host B answers with its MAC address.
  4. A stores the pair in its ARP cache.
  5. Frames can now be addressed directly to B.

Where it is used

Every Ethernet or Wi-Fi host uses ARP before sending to a local IP or to its default gateway. The arp command shows the table, which helps debug connectivity.

Common pitfall

ARP has no authentication, so attackers can spoof replies and perform man-in-the-middle attacks known as ARP poisoning. Stale entries after hardware changes cause temporary outages. Duplicate IPs create flapping entries. Use dynamic ARP inspection on managed switches and static entries for critical devices.

Where it fits in the course

ARP is topic 27 of 50 in Computer Networks, rated Intermediate. It follows CRC. The next topic is ICMP and Ping.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Computer Networks quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is ARP?
Protocol mapping IP addresses to MAC addresses. It belongs to Computer Networks and is an intermediate-level topic.
Where do people use ARP?
Every Ethernet or Wi-Fi host uses ARP before sending to a local IP or to its default gateway.
What is the most common mistake with ARP?
ARP has no authentication, so attackers can spoof replies and perform man-in-the-middle attacks known as ARP poisoning.
What should I learn before this topic?
Start with CRC, the topic just before it in Computer Networks.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Computer Networks quiz.
Is ARP hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn ARP?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to ARP?
CRC, ICMP and Ping, Error Detection and Correction are closely connected. The full list is in the related topics section of this page.
What should I study after ARP?
Continue with ICMP and Ping, the next topic in Computer Networks.

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