DRUVEXAA EDUCATION PRESENTS

ARP

ADDRESS RESOLUTION PROTOCOL

Every device has two addresses. The IP address is like a person’s name. The MAC address is like their exact house address.
ARP is the helper that asks: “I know your name… but where is your house?”

🖱️ Drag to orbit • Scroll to zoom — anytime during the simulation

ARP SIMULATOR
DRUVEXAA EDUCATION

💻 PC1 · ARP CACHE (memory)

IP ADDRESSMAC ADDRESS
— empty (nothing learned yet) —

🔀 LAYER-2 SWITCH · MAC TABLE

MAC ADDRESSPORT
— switch has not learned any device yet —
PACKET LEGEND
ARP Request (Broadcast — asks everyone)
ARP Reply (Unicast — private answer)
Data Frame (the real message)
Router / Gateway action
📦 ETHERNET FRAME — THE COURIER ENVELOPE
TO (Dest MAC)—
FROM (Src MAC)—
TYPE—
INSIDE 💌—
SEAL (FCS)CRC checksum ✓ not damaged
Like a courier parcel: the envelope (MACs) can change at every hop, but the letter inside stays the same.
—
Press Start to begin the simulation.
📖 ARP — THE COMPLETE CONCEPT

1️⃣ WHAT IS ARP?

ARP (Address Resolution Protocol) is a simple question-and-answer system used inside a local network. Its only job: convert an IP address into a MAC address.

🚚 Think of a courier company:

  • The IP address is like the receiver’s name — easy to remember, can change if the person moves.
  • The MAC address is like the exact house address — burnt into the network card at the factory, never changes.
  • A courier cannot deliver a parcel with only a name. He must find the house address first. ARP is the phone call that asks for it.

2️⃣ IP vs MAC — QUICK COMPARISON

IP ADDRESS
MAC ADDRESS
Example
192.168.1.10
AA:AA:AA:00:00:01
Works like
A person’s name
The house address
Given by
Network / router (can change)
Factory (permanent)
Used for
End-to-end journey
Next-hop delivery only
OSI Layer
Layer 3 (Network)
Layer 2 (Data Link)

3️⃣ THE ETHERNET FRAME — A COURIER PARCEL 📦

Every message on a local network travels inside an Ethernet frame. The ARP request itself is a small letter placed inside this parcel:

DESTINATION MAC
Where the parcel goes.
Broadcast = FF:FF:FF:FF:FF:FF
SOURCE MAC
Return address
of the sender
TYPE
0x0806 = ARP
0x0800 = IP data
PAYLOAD 💌
The letter inside:
ARP message or data
FCS (SEAL)
Checksum — proves the
parcel was not damaged

🔑 Golden idea: when the parcel crosses a router, the router opens it, keeps the same letter (IP packet), puts it into a new envelope with new MAC addresses, and sends it on. MACs change hop-by-hop. IPs stay the same end-to-end.

4️⃣ WHAT DOES THE SWITCH DO?

The devices are connected through a Layer-2 Switch (works with MAC addresses only — it never reads IP addresses).

  • 📥 The switch learns: when a frame arrives, it notes “this Source MAC lives on this port” in its MAC address table.
  • 📢 A broadcast frame (TO = FF:FF:FF:FF:FF:FF) is flooded out of every port — everyone receives it.
  • 🎯 A unicast frame is sent out of only one port — the one where that MAC lives. Fast and private.

5️⃣ ARP DECISION FLOWCHART

🖥️ PC1 wants to send data to a target IP address
▼
❓ Is the target IP inside MY network?
(PC compares using its subnet mask)
YES — SAME NETWORK
Is the target’s MAC already in my ARP cache?
▼ NO
📢 Broadcast ARP REQUEST:
“Who has this IP? Tell me your MAC!”
▼
📬 Target replies (unicast) with its MAC → PC1 saves it in cache
▼
🚀 Send data frame DIRECTLY to the target’s MAC
NO — DIFFERENT NETWORK
🚪 Use the DEFAULT GATEWAY.
ARP for the router’s MAC (not the target’s!)
▼
📦 Send frame to the router:
Dest IP = target • Dest MAC = router
▼
🔁 Router repeats ARP on the far network, finds the target’s MAC, and rewrites the envelope
▼
🎉 Frame delivered on the remote network
▼
💾 Every answer is saved in the ARP cache — next time, no question needed (until the entry expires, usually after a few minutes)

6️⃣ FIVE FACTS TO REMEMBER

  • 📢 ARP Request is always a broadcast — every device on the local network hears it.
  • 📬 ARP Reply is always a unicast — a private answer straight back to the asker.
  • 🚧 ARP never crosses a router. Routers stop broadcasts. That is why we need a default gateway.
  • 💾 The ARP cache avoids repeating the same question — it makes the network fast.
  • ✉️ In a data frame to a remote network: Destination IP = final target, but Destination MAC = only the next hop.