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Troubleshoot — two PCs on one switch that cannot talk
Enterprise Beginner 4 devices 3 checked tasks 100 points Free lab
Two PCs in the same subnet on the same switch, the right addresses, cables up, no firewall — and no ping. Layer 1 is fine and layer 3 is fine; look at layer 2 and ask which VLAN each port is in. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This is a free lab: it stays open on the free plan. It needs an account, which keeps your progress.
The scenario
Two PCs in the same subnet on the same switch, the right addresses, cables up, no firewall — and no ping. Layer 1 is fine and layer 3 is fine; look at layer 2 and ask which VLAN each port is in.
Your role: a network engineer on the enterprise team. Level: Beginner Done when: all 3 goals show as passed (100 points).
Network status
Devices: 1 switch, 3 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. SW1 starts with and VLAN 10, VLAN 20 already configured. PC1 is a host with 10.0.10.11/24. PC2 is a host with 10.0.10.12/24. PC3 is a host with 10.0.10.13/24. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
All three user ports are in VLAN 10 (40 pts) PC1 reaches PC2 (30 pts) PC3 reaches PC2 (30 pts) The activity, in order
All three user ports are in VLAN 10 Why: A VLAN is a separate broadcast domain; the port’s VLAN decides which devices can talk without a router. Verify with: show vlan brief PC1 reaches PC2 Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.10.12 PC3 reaches PC2 Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.10.12 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Assuming a green link light means connectivity Common mistake: Fixing the PC’s address when the port is the problem Common mistake: Assigning the VLAN before it exists on the switch Why this matters
Layer 1 and 3 can be perfect and two hosts still cannot talk because their ports are in different VLANs — different broadcast domains. show vlan brief and show interfaces status tell you in seconds.
What you will be able to do
Read the VLAN membership of access ports
Explain why two hosts in one subnet but different VLANs cannot communicate
Move the port and verify
The network
Catalyst 2960-24TT, PC — 4 devices in all, already cabled.
Fa0/1 Fa0 Fa0/2 Fa0 Fa0/3 Fa0 SW1 PC1 10.0.10.11/24 PC2 10.0.10.12/24 PC3 10.0.10.13/24 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
SW1 Catalyst 2960-24TT configure it PC1 PC 10.0.10.11 / 255.255.255.0 PC2 PC 10.0.10.12 / 255.255.255.0 PC3 PC 10.0.10.13 / 255.255.255.0
Cabling
SW1 Fa0/1 ↔ PC1 Fa0 SW1 Fa0/2 ↔ PC2 Fa0 SW1 Fa0/3 ↔ PC3 Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
SW1 vlan 10
name USERS
vlan 20
name GUEST
exit
interface Fa0/1
switchport mode access
switchport access vlan 10
exit
interface Fa0/2
switchport mode access
switchport access vlan 20
exit
interface Fa0/3
switchport mode access
switchport access vlan 10
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 All three user ports are in VLAN 10 40 On SW1: show vlan brief — Fa0/1 is an access port in VLAN 10. and On SW1: show vlan brief — Fa0/2 is an access port in VLAN 10. and On SW1: show vlan brief — Fa0/3 is an access port in VLAN 10. 2 PC1 reaches PC2 30 On PC1: ping 10.0.10.12 must succeed. 3 PC3 reaches PC2 30 On PC3: ping 10.0.10.12 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Assuming a green link light means connectivity
Fixing the PC’s address when the port is the problem
Assigning the VLAN before it exists on the switch
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
SW1 interface Fa0/2
switchport access vlan 10
exit Where this is examined
Network+ (switching problems), CCNA (VLANs and trunking).
This lab practises these topics in the zNetLaB certification courses:
Related labs
Where to start
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