What ARP Learned

Lesson 7 of 35 in Coddy's LAN Labs course.

Hosts are configured with IP addresses, but on a LAN, frames are delivered to MAC addresses: the hardware address burned into each network card. ARP is how a host discovers the MAC behind an IP: it asks the whole segment "who has 192.168.1.20?" and the owner answers.

The answers are cached in the host's ARP table. Before any traffic the table is empty; after one ping it holds the neighbor's MAC. netlab show arp HOST displays it. The lab's MACs start with 02:c0:dd, netlab's own vendor prefix.

Challenge

Beginner

Show pc1's ARP table, ping pc2 from pc1, then show the ARP table again.

Expected output:

pc1: ARP table is empty (no traffic yet)
pc1 -> pc2: reply from 192.168.1.20 (ttl=64)
ADDRESS       MAC
192.168.1.20  02:c0:dd:a0:9f:64

Try it yourself

Terminal

All lessons in LAN Labs

1Meet the Lab

Your First Lab Cables Wires Are Not Enough Recap - Build a Lab

4Routers and Gateways

Two Networks The Default Gateway Both Sides Need a Way Back Routing Tables Recap - Connect the Floors

7Growing the Office

The Plan Sales Comes Online Engineering Joins Gateways Everywhere Recap - The Printer

2Addresses on the LAN

Giving a Host an Address The First PingWhat ARP LearnedThe Switch Learns Too Recap - First Conversation

5Troubleshooting I

The Unplugged Host The Missing Gateway The Typo in the Mask Recap - Two Faults

8Capstone - Fix the Branch

Survey the Damage Fix the Address Clash Masks and Gateways Recap - Prove It Works

3Masks Matter

Wrong Mask, No Reply Same Switch, Other Subnet Let check Do the Looking Recap - Diagnose the Pair

6Troubleshooting II

Two Devices, One Address The Network Address Trap Gateway on the Wrong Subnet Recap - The Silent Router

Practice on your own: Terminal playground