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Troubleshoot — a subnet mask that is almost right
Enterprise Beginner 5 devices 3 checked tasks 100 points Free lab
PC2 answers pings from PC1 sometimes and never reaches anything beyond the switch. Its address looks right and so does its gateway. Read the mask the way the PC reads it — which network does it think it is on, and is the gateway on that network? 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
PC2 answers pings from PC1 sometimes and never reaches anything beyond the switch. Its address looks right and so does its gateway. Read the mask the way the PC reads it — which network does it think it is on, and is the gateway on that network?
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 router, 1 switch, 2 PCs, 1 server, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. R1 starts with addresses (G0/0 10.0.1.1/24, G0/1 10.0.2.1/24). PC1 is a host with 10.0.1.11/24, gateway 10.0.1.1. PC2 is a host with 10.0.1.200/25, gateway 10.0.1.1. Server is a host with 10.0.2.10/24, gateway 10.0.2.1. SW1 starts with an empty configuration. 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
PC2’s mask puts its gateway on its own subnet (255.255.255.0) (40 pts) PC2 reaches its gateway (30 pts) PC2 reaches the server (30 pts) The activity, in order
PC2’s mask puts its gateway on its own subnet (255.255.255.0) Why: Reading a show command is the core skill of troubleshooting: the output is the truth, not your configuration. Verify with: ipconfig PC2 reaches its gateway Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.1.1 PC2 reaches the server Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.2.10 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: Checking only the address, not the mask Common mistake: Mixing up /25 and /24 boundaries when working in your head Common mistake: Fixing the mask on one host but not the DHCP scope Why this matters
A mask that is almost right makes the host think its gateway is on another network, so it never ARPs for it. “It looks fine” is the classic way a /25 survives for months.
What you will be able to do
Work out which network a host believes it is on from its address and mask
Check whether the gateway is inside that network
Correct the mask and re-test
The network
ISR 2911, Catalyst 2960-24TT, PC, Server — 5 devices in all, already cabled.
Fa0/1 Fa0 Fa0/2 Fa0 Gi0/1 Gi0/0 10.0.1.1/24 Gi0/1 10.0.2.1/24 Fa0 R1 SW1 PC1 10.0.1.11/24 · gw 10.0.1.1 PC2 10.0.1.200/25 · gw 10.0.1.1 Server 10.0.2.10/24 · gw 10.0.2.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R1 ISR 2911 configure it SW1 Catalyst 2960-24TT configure it PC1 PC 10.0.1.11 / 255.255.255.0 gw 10.0.1.1 PC2 PC 10.0.1.200 / 255.255.255.128 gw 10.0.1.1 Server Server 10.0.2.10 / 255.255.255.0 gw 10.0.2.1
Cabling
SW1 Fa0/1 ↔ PC1 Fa0 SW1 Fa0/2 ↔ PC2 Fa0 SW1 Gi0/1 ↔ R1 Gi0/0 R1 Gi0/1 ↔ Server 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:
R1 interface g0/0
ip address 10.0.1.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 10.0.2.1 255.255.255.0
no shutdown
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 PC2’s mask puts its gateway on its own subnet (255.255.255.0) 40 On PC2: ipconfig output matches Subnet Mask[ .]*: 255\.255\.255\.0. 2 PC2 reaches its gateway 30 On PC2: ping 10.0.1.1 must succeed. 3 PC2 reaches the server 30 On PC2: ping 10.0.2.10 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
Checking only the address, not the mask
Mixing up /25 and /24 boundaries when working in your head
Fixing the mask on one host but not the DHCP scope
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:
PC2 netsh interface ip set address "Ethernet" static 10.0.1.200 255.255.255.0 10.0.1.1 Where this is examined
Network+ (subnetting, troubleshooting), CCNA (IPv4 addressing).
This lab practises these topics in the zNetLaB certification courses:
Related labs
Where to start
The lessons all guided labs plans
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