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Network Troubleshooting Lab

Diagnose and resolve connectivity issues using simulated ping, traceroute, nslookup, and more

75 minutes Intermediate 6 Scenarios
R

Troubleshooting Toolkit Reference

Available commands in the simulator

ping

ping <host>

Test Layer 3 reachability. Sends ICMP echo requests to a target.

traceroute

traceroute <host>

Show the path packets take. Reveals where connectivity breaks.

nslookup

nslookup <domain>

Query DNS. Check if name resolution is working correctly.

ipconfig

ipconfig

Show local IP configuration: address, mask, gateway, DNS.

arp

arp -a

Show the ARP table. Maps IP addresses to MAC addresses.

netstat

netstat

Show active connections and listening ports on the local host.

Troubleshooting Method: Always work bottom-up through the OSI model. (1) Check physical/link layer (cable, NIC). (2) Check IP config (ipconfig). (3) Ping default gateway. (4) Ping remote IP. (5) Check DNS (nslookup). (6) Trace the path (traceroute).
1

The Missing Gateway

Beginner
Ticket: A user reports they cannot access any websites or reach any servers outside their subnet. Other users on the same switch are working fine.
User PC (10.0.1.50/24) | [Switch] --- [Router: 10.0.1.1] --- [Internet] | Other PCs (working fine)
user-pc
Type commands to diagnose the issue. Try: ipconfig, ping, traceroute, nslookup, arp
user-pc>
2

Names Without Faces

Beginner
Ticket: A user says "the internet is down." They cannot open any websites by name, but you suspect the issue might be more specific than total connectivity loss.
User PC (192.168.1.100/24) | [Switch] --- [Router: 192.168.1.1] --- [ISP DNS: 8.8.8.8] | [Web Server: 93.184.216.34 = example.com]
workstation
User says "internet is down." Investigate systematically.
workstation>
3

The Narrowed Horizon

Intermediate
Ticket: A workstation can ping some hosts on the local network (10.0.1.1 through 10.0.1.15) but cannot reach 10.0.1.50 or anything beyond the local subnet. The switch and router are confirmed working for other hosts.
Problem PC (10.0.1.5/???) Server (10.0.1.50/24) | | [Switch] -------- [Router: 10.0.1.1/24] --- [Internet] | Working PCs (10.0.1.20-30, all /24, working fine)
problem-pc
This PC can reach some hosts but not others. Investigate.
problem-pc>
Insight: A /28 mask means the PC thinks its network is 10.0.1.0 - 10.0.1.15. Any IP above .15 appears to be on a "different network," so the PC tries to route through its gateway instead of reaching them directly. If the gateway also seems unreachable due to mask confusion, everything breaks.
4

The Black Hole

Intermediate
Ticket: Users on the 10.0.1.0/24 network can reach the internet, but cannot access the branch office server at 172.16.5.10. The branch office confirms their server is up and reachable locally.
HQ Network Branch Office 10.0.1.0/24 172.16.5.0/24 | | [HQ Router: 10.0.1.1] --- [ISP] --- [Branch Router: 172.16.5.1] | [Server: 172.16.5.10]
hq-workstation
Cannot reach branch server 172.16.5.10. Internet works fine.
hq-workstation>
5

The Impostor

Advanced
Ticket: A user reports intermittent connectivity. Sometimes pings work, sometimes they do not. The connection drops and recovers randomly every few seconds. Another user nearby has no issues.
User A PC (192.168.1.25/24) User B PC (192.168.1.25/24) <-- CONFLICT! | | [Fa0/1] [Fa0/5] | | [============ Switch ============] | [Router: 192.168.1.1]
user-a-pc
Intermittent connectivity. Pings sometimes succeed, sometimes fail.
user-a-pc>
Duplicate IP Detection: When two hosts share an IP, the switch ARP table oscillates between their MAC addresses. Intermittent failures are the hallmark. Windows will pop a "duplicate IP" warning, but not always immediately. Use arp -a and watch the MAC for the conflicting IP change between checks.
6

The Perfect Storm

Advanced
Ticket: A newly provisioned server (10.0.2.100) in the DMZ cannot reach anything. It was just racked and cabled. The network team says the switch port is up. Systematically identify ALL misconfigurations (there are multiple).
New Server (10.0.2.100/24) | [DMZ Switch] --- [Firewall: 10.0.2.1] --- [Core Router] --- [Internet] | [LAN: 10.0.1.0/24]
dmz-server
New server cannot reach anything. Find ALL misconfigurations.
dmz-server>
Lesson: New server builds often have multiple configuration errors stacked together. A systematic bottom-up approach catches them all. Check IP, mask, gateway, and DNS independently -- never assume only one thing is wrong.