Two Devices, One Address

Lesson 23 of 35 in Coddy's LAN Labs course.

An IP address must be unique on its network. When two devices claim the same one, ARP gets two answers to "who has 192.168.1.20?" and frames go to whichever answered last. Users experience it as a machine that works, then vanishes, then works again.

netlab refuses to guess and reports the clash: 192.168.1.20 is used by both pc2 and pc4. netlab check flags the same thing as an ERROR without any traffic, which is why it belongs at the start of every troubleshooting session.

Challenge

Easy

Ping pc2 from pc1, run the check, move the newcomer pc4 to 192.168.1.40/24, and ping pc2 again.

Expected output:

pc1 -> pc2: unreachable (192.168.1.20 is used by both pc2 and pc4)
ERROR 192.168.1.20 is used by pc2 and pc4
Lab check: 1 problem(s) found
pc4 eth0: 192.168.1.40/24 (mask 255.255.255.0, network 192.168.1.0/24)
pc1 -> pc2: reply from 192.168.1.20 (ttl=64)

Try it yourself

Terminal