We need ARP (Address Resolution Protocol) to enable communication between devices on a local network. Specifically, ARP is used to map an IP address (logical address) to a MAC address (physical address), which is necessary for data transmission at the data link layer.
ARP is a Layer 2 protocol used in IPv4 networks to map a known IP address to its corresponding MAC (Media Access Control) address. While IP addresses help devices find each other logically, MAC addresses are what Ethernet networks use to actually deliver packets. ARP acts as the translator between these two.
π€ Why ARP is needed:
IP works at Layer 3 (Network Layer) but to actually send data over Ethernet (Layer 2), the system needs the MAC address of the destination device.
When a device wants to communicate with another device on the same network, it checks its ARP cache. If it doesn't know the MAC address of the destination IP, it sends out an ARP request.
The device with the matching IP responds with its MAC address, allowing the sender to create a proper Ethernet frame.
βββββββββββββββββββββββ
β PC-B (Receiver) β
β IP: 192.168.1.20 β
β MAC: BB-BB-BB-BB β
βββββββββββ²ββββββββββββ
β²
[ARP Reply: 192.168.1.20 is at BB-BB-BB-BB]
β²
[ARP Request: Who has 192.168.1.20? Tell 192.168.1.10]
β²
βββββββββββ΄ββββββββββββ
β PC-A (Sender) β
β IP: 192.168.1.10 β
β MAC: AA-AA-AA-AA β
βββββββββββββββββββββββπ―οΈWithout ARP:
The system wouldn't be able to figure out where to send the packet at the data link layer, even if it knows the IP.
π Does ARP help when sending a packet to the internet?
Yes β but only for the first hop, i.e., your default gateway (usually a router).
πΈ Hereβs how it works:
You want to send a packet to an external IP, say 8.8.8.8.
Your computer checks its routing table and sees that this IP is not on the local network, so it must go via the default gateway (router).
But to send the packet to the router, your machine needs the MAC address of that gateway.
Thatβs where ARP is used: Your machine sends an ARP request for the gatewayβs IP β the router replies with its MAC address.
Your system then encapsulates the IP packet for 8.8.8.8 inside an Ethernet frame addressed to the routerβs MAC.
After that: The packet travels over the internet β and each router along the way repeats this process, but ARP is only used within local networks (LAN segments).
βββββββββββββββββββββββ
β Internet (8.8.8.8) β
βββββββββββ²ββββββββββββ
β
βββββββββββββββ΄βββββββββββββββ
β Default Gateway (Router) β
β IP: 192.168.1.1 β
β MAC: BB-BB-BB-BB-BB-BB β
βββββββββββββββ²βββββββββββββββ
β²
[ARP Reply: 192.168.1.1 is at BB]
β²
[ARP Request: Who has 192.168.1.1?]
β²
βββββββββββββββ΄βββββββββββββββ
β Your Computer (PC) β
β IP: 192.168.1.10 β
β MAC: AA-AA-AA-AA-AA-AA β
βββββββββββββββββββββββββββββββΎοΈ Why IP address cannot deliver the packet to the system ?
An IP address alone cannot deliver a packet to a system on a local network because Ethernet (and most local networks) operate at a lower layer β theβ―Data Link Layer (Layer 2) β which uses MAC addresses for actual delivery.
Here's a breakdown of why:
𧱠The Layer Mismatch:
- IP addresses operate at theβ―Network Layer (Layer 3) β theyβre logical addresses used for routing across networks.
- But Ethernet, which is the most common local network protocol, delivers packets at theβ―Data Link Layer (Layer 2) β and it needs a physical address, i.e., the MAC address.
π So what happens?
When you try to send a packet to an IP address on your LAN:
- Your system knows the destination IP (e.g., 192.168.1.20).
- It checks the ARP cache to see if it already knows the MAC address for that IP.
- If not, it sends an ARP Request: βWho has 192.168.1.20?β
- The device with that IP responds with its MAC.
- Now, your system wraps the IP packet inside an Ethernet frame addressed to the MAC address and sends it over the network.
Without that MAC address, the Ethernet frame would have no target β and the packet would be undeliverable.
π Chart showing how different layers of the OSI model use different types of addresses to deliver data:
+----------------+----------------------------+---------------------------+
| OSI Layer | Address Type | Example |
+----------------+----------------------------+---------------------------+
| Application | Port Number | Port 80 (HTTP) |
| | | Port 443 (HTTPS) |
+----------------+----------------------------+---------------------------+
| Transport | Port Number | Src: 54321 β Dst: 80 |
+----------------+----------------------------+---------------------------+
| Network | IP Address (Logical) | 192.168.1.10 |
| | | 8.8.8.8 |
+----------------+----------------------------+---------------------------+
| Data Link | MAC Address (Physical) | AA-AA-AA-AA-AA-AA |
| | | BB-BB-BB-BB-BB-BB |
+----------------+----------------------------+---------------------------+