Build an Office Network in Packet Tracer

Configure routing, DHCP, and subnetting for a two-department office network.

Introduction

30 Second Summary

Every time you send a message to a coworker in a different part of the office, your data travels through a chain of networking devices to get there. Understanding how that journey works is the foundation of every networking career.

In this project, you will build a two-department office network from scratch in Cisco Packet Tracer. You will connect PCs, configure a router, set up automatic IP addressing, and deliberately break the network and fix it using the same troubleshooting methodology tested on the CompTIA Network+ exam.

What You'll Build

Picture typing a ping command on a Sales PC and watching replies come back from an Engineering PC on a completely different subnet, all routed through a network you built and configured yourself.

By the end of this project, you'll have:

A working two-department office network where Sales PCs can ping Engineering PCs across different subnets, proving the router is forwarding traffic correctly.
Four PCs that automatically receive their IP addresses from the router via DHCP, with zero manual configuration.
A diagnosed and fixed network failure that you broke on purpose, applying the same troubleshooting methodology tested on the Network+ exam.
Secret Mission: Add a third department with its own subnet, router interface, and DHCP pool so all three departments can communicate.

Are there any prerequisites?

None. This project includes full setup instructions for every tool you need, including Cisco Packet Tracer. No prior networking experience is required.

Before We Start

Before you start placing routers and switches, take a moment to lock in what you're about to build and why it matters.

Install Cisco Packet Tracer and Create Your Workspace

Building a real network requires physical routers, switches, and cables. That equipment costs thousands of dollars and fills an entire server room.

Cisco Packet Tracer lets you simulate all of it on your computer for free. In this step, you will create a Cisco Networking Academy account, install Packet Tracer, and place your first device on the workspace canvas.

In this step, get ready to:

Create a Cisco Networking Academy account and download Packet Tracer.
Launch Packet Tracer and log in with your credentials.
Place a 2911 router on the workspace to confirm everything works.

Create a Cisco Networking Academy account and download Packet Tracer

Packet Tracer is free, but it requires a Cisco Networking Academy account to download and use. You will create the account first, then grab the installer.

Open https://www.netacad.com/cisco-packet-tracer in your browser.
Click the sign-up option and create a free Cisco Networking Academy account.
Once you are logged in, navigate to the Resource Hub at https://www.netacad.com/resources/lab-downloads.
Download Cisco Packet Tracer for Windows 64-bit.

Once the download finishes, run the installer. It walks you through a series of screens.

Double-click the downloaded installer file to launch it.
Read through the License Agreement, select I accept the agreement, and click Next.
In the Select Destination Location window, accept the default location and click Next.
In the Select Start Menu Folder window, click Next.
In the Select Additional Tasks window, click Next.
Click Install and wait for the installation to complete.

On macOS or Linux?

Packet Tracer is also available for macOS and Linux on the same Resource Hub page. Download the version that matches your operating system and follow your platform's installer prompts. The rest of this project works identically once Packet Tracer is open.

Download or install not working?

Make sure you are signed in to your Cisco Networking Academy account before visiting the Resource Hub. The download links only appear for logged-in users.

Check that your system meets the minimum requirements: Windows 10 or 11, 4 GB RAM, and 1.4 GB of free disk space.

Help me troubleshoot my Packet Tracer installation

Launch Packet Tracer and log in

With Packet Tracer installed, you need to launch it and authenticate with your Cisco Networking Academy credentials. Packet Tracer requires a login every time it opens.

Press Cmd+Space (macOS) or the Windows key (Windows) to open your search bar.
Type Cisco Packet Tracer and press Enter to open the application.

What prompts will I see on first launch?

Packet Tracer shows a couple of prompts the first time it opens. A dialog asks whether you want to run multi-user mode, and Windows may show a Security Alert about network access. Both are covered in the bullets below.

When asked "Would you like to run multi-user when the application starts?", click No.
If a Windows Security Alert appears, click Allow access.
Click the LOGIN button in the Packet Tracer window.
A browser tab opens. Sign in with the same email and password you used to create your Cisco Networking Academy account.
Once you see the message "You have successfully logged in to Cisco Packet Tracer", close the browser tab and return to Packet Tracer.

You should now see the Packet Tracer workspace. It has a large blank canvas in the center and a device toolbar running along the bottom of the screen.

Login not completing?

Make sure you are using the same email and password you registered with. If the browser tab does not open after clicking LOGIN, check that your default browser is not blocking pop-ups from Packet Tracer.

Help me log in to Cisco Packet Tracer

Place a router and verify your workspace

The device toolbar at the bottom of the screen is where you find all the network equipment you can add to your topology. Let's place your first device to confirm Packet Tracer is working correctly.

Click Routers in the device category bar at the bottom-left of the screen.
Find the 2911 router in the device list that appears to the right.
Drag the 2911 router onto the center of the workspace canvas.

Before you look at the result, what do you think a router icon looks like on a network diagram?

You should see a Cisco 2911 router icon on the canvas with the label "Router0" underneath it. That is your first piece of network equipment.

What is a router?

A router is a device that forwards data between different networks (called subnets). In this project, the 2911 router will connect your Sales and Engineering departments so they can communicate with each other.

The 2911 has three GigabitEthernet interfaces, which means it can connect up to three separate networks. That is perfect for the two departments you will build, plus a third in the Secret Mission.

Don't see the router on the canvas?

Make sure you clicked Routers in the device category bar, not Switches or End Devices. The 2911 appears in the device list to the right of the category bar.

If you placed the wrong device by mistake, right-click it on the canvas and select Delete. Then drag the correct 2911 router from the list.

Help me place a router in Packet Tracer

Your simulation environment is ready to go. Next up, you will connect two PCs to a switch and get your very first ping working.

Build a Single-Department LAN and Get Your First Ping

Your Packet Tracer workspace is ready and you have a router sitting on the canvas. Now it is time to build something that actually works.

Before tackling routing between departments, you need to prove the basics. Two PCs, one switch, one subnet. If they can ping each other, Layer 2 connectivity is working. This is the baby version of the two-department office network you are building.

In this step, get ready to:

Place a switch and two PCs on the workspace and cable them together.
Assign static IP addresses to both PCs.
Send your first ping and verify the PCs can talk to each other.

Place devices and connect them

A switch connects devices on the same local network so they can communicate. You will add one switch and two PCs, then cable them together.

In the device category bar at the bottom-left of the workspace, click Network Devices (the icon that looks like a group of devices).
Click Switches in the subcategory row that appears.
Drag a 2960 switch onto the workspace to the right of your router.

You should see a switch icon labelled Switch0 appear on the canvas next to your router.

Click End Devices in the device category bar at the bottom-left.
Drag a PC onto the workspace below the switch.
Drag a second PC onto the workspace to the right of the first one.

You should see PC0 and PC1 on the canvas alongside your switch and router. Now you need to cable the PCs to the switch.

Click Connections in the device category bar (the lightning bolt icon).
Select Copper Straight-Through (the solid black line).
Click on PC0 and select FastEthernet0 from the port list.
Click on Switch0 and select FastEthernet0/1 from the port list.

A cable now connects PC0 to the switch. Repeat the same process for PC1.

With Copper Straight-Through still selected, click on PC1 and select FastEthernet0.
Click on Switch0 and select FastEthernet0/2.

Wait a few seconds. The link lights on both cables should turn green, which means the physical connections are active.

What is a Copper Straight-Through cable?

Copper Straight-Through cables connect unlike devices together, such as a PC to a switch or a switch to a router. Packet Tracer simulates this just like real-world Ethernet cabling.

Link lights still orange?

Orange link lights mean the connection is still initialising. Wait about 30 seconds for the switch ports to come up. If they stay orange, check that you used Copper Straight-Through (not Crossover or another cable type) and that each cable connects to the correct ports.

Why are my link lights not turning green in Packet Tracer?

Assign static IP addresses to both PCs

The PCs are physically connected, but they cannot communicate yet. Each device needs an IP address so it knows where to send traffic. You will assign addresses on the same subnet so both PCs recognise each other as neighbours.

Click on PC0 to open its configuration window.
Select the Desktop tab.
Click IP Configuration.
Make sure Static is selected (not DHCP).
Enter 192.168.1.10 in the IP Address field.
Enter 255.255.255.0 in the Subnet Mask field.
Leave the Default Gateway field blank.

You should see the values you entered reflected in the IP Configuration panel. Close the PC0 window.

Click on PC1 to open its configuration window.
Select the Desktop tab.
Click IP Configuration.
Make sure Static is selected.
Enter 192.168.1.11 in the IP Address field.
Enter 255.255.255.0 in the Subnet Mask field.
Leave the Default Gateway field blank.

Confirm the IP address and subnet mask appear correctly in the panel, then close the PC1 window.

What does the subnet mask do?

A subnet mask tells a device which part of an IP address identifies the network and which part identifies the individual device. With a mask of 255.255.255.0, the first three numbers (192.168.1) are the network address and the last number is the device.

Because both PCs share the same network portion (192.168.1), they know they are on the same local network and can reach each other directly through the switch.

IP address not saving?

Make sure the Static radio button is selected in the IP Configuration panel. If DHCP is selected, the fields will not accept manual input.

Why can't I enter a static IP address on my PC in Packet Tracer?

Ping between PCs to verify Layer 2 connectivity

Both PCs have IP addresses on the same subnet. The moment of truth is a ping. This sends a small test packet from one PC to the other and waits for a reply.

Click on PC0 to open its configuration window.
Select the Desktop tab.
Click Command Prompt.

Before you run the next command, what do you think will happen? Will PC0 get a reply from PC1, or will the ping fail?

Test connectivity to PC1 by typing this command and pressing Enter:

ping 192.168.1.11

You should see replies from 192.168.1.11 with 0% packet loss. Your two PCs can talk to each other through the switch.

First ping timed out?

It is completely normal for the very first ping packet to time out. When PC0 pings PC1 for the first time, it has to perform an ARP lookup to discover PC1's hardware address. That lookup takes long enough for the first packet to expire. The remaining three pings should all succeed.

All four pings failed?

Double-check that both PCs are on the same subnet. PC0 should be 192.168.1.10 and PC1 should be 192.168.1.11, both with subnet mask 255.255.255.0.
Verify the cable link lights are green on both connections. Orange or red lights mean the physical link is not active.
Make sure you typed the ping command from PC0's Command Prompt, not from the router CLI.

My ping between two PCs on the same switch is failing. Help me troubleshoot.

Why leave the default gateway blank?

A default gateway tells a device where to send traffic that is destined for a different network. Right now both PCs are on the same network, so they do not need one. You will add a default gateway in the next step when you connect a second department on a different subnet.

Your first LAN is live. Two PCs, one switch, one subnet, and a successful ping proving it all works. Next up, you will add a second department on a different subnet and discover why a router is the missing piece.

Add a Second Department and Configure Routing

Your Sales network is live. PC0 and PC1 can ping each other on the same subnet, proving that your cabling and switching work.

Real offices have multiple departments on separate subnets. In this step, you'll add an Engineering department on its own subnet and configure the router to connect both networks so traffic can flow between them.

In this step, get ready to:

Add a second switch and two PCs for the Engineering department.
Configure the router to route traffic between both subnets.
Set default gateways on all PCs and verify cross-subnet connectivity.

Build the Engineering department

Engineering needs its own network segment. You'll build it the same way you built the Sales LAN: a switch, two PCs, and cables.

Click Switches in the device category bar at the bottom of the workspace.
Drag a 2960 switch onto the canvas to the right of your existing setup. Packet Tracer names it Switch1 automatically.
Click End Devices in the device category bar.
Drag two PCs onto the canvas below Switch1. Packet Tracer names them PC2 and PC3.
Click the Connections category (the lightning bolt icon) in the device toolbar.
Select Copper Straight-Through.
Click PC2's FastEthernet0 port, then click Switch1's FastEthernet0/1 port.
Click PC3's FastEthernet0 port, then click Switch1's FastEthernet0/2 port.

The link lights on both connections turn green after a few seconds. This confirms the physical links are up.

Click on PC2, go to the Desktop tab, and select IP Configuration.
Set the IP address to 192.168.2.10 and the subnet mask to 255.255.255.0. Leave the default gateway blank for now.
Click on PC3, go to the Desktop tab, and select IP Configuration.
Set the IP address to 192.168.2.11 and the subnet mask to 255.255.255.0. Leave the default gateway blank.

Verify the Engineering LAN works by clicking on PC2, going to Desktop, and opening Command Prompt.
Test connectivity between PC2 and PC3 by running this command:

ping 192.168.2.11

You should see replies from 192.168.2.11 with 0% packet loss. The first ping may time out due to ARP resolution, which is normal.

What is ARP resolution?

When a device sends its first ping to a new address, it needs to discover the destination's physical (MAC) address using the Address Resolution Protocol. This lookup adds a brief delay that can cause the first packet to time out. Every ping after that succeeds because the address is now cached.

Not seeing replies from PC3?

Double-check that PC2's IP is 192.168.2.10 and PC3's IP is 192.168.2.11. Both must share the same subnet mask 255.255.255.0.
Confirm the cables go from each PC's FastEthernet0 to Switch1's FastEthernet0/1 and FastEthernet0/2 ports. Both link lights should be green.
If link lights are still amber, wait 30 seconds for Spanning Tree Protocol to finish converging, then try again.

Why can't my Engineering PCs ping each other?

Connect both switches to the router and configure router interfaces

Your two departments are now separate islands. PC0 can reach PC1, and PC2 can reach PC3, but Sales cannot reach Engineering because they are on different subnets.

The router already sitting on your canvas is the bridge between them. You'll cable each switch to a different router port, then configure those ports with IP addresses using the Cisco IOS CLI.

Select Copper Straight-Through from the Connections category again.
Click any available port on Switch0 (the Sales switch), then click the router's GigabitEthernet0/0 port.
Click any available port on Switch1 (the Engineering switch), then click the router's GigabitEthernet0/1 port.

You'll see link lights appear on both connections. The router-side lights stay red for now because the interfaces are shut down by default.

Click on the router and go to the CLI tab.
If you see a prompt asking if you'd like to enter the initial configuration dialog, type no and press Enter. Press Enter again to get to the Router> prompt.
Configure both router interfaces by entering these commands:

enable
configure terminal
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 192.168.2.1 255.255.255.0
no shutdown
exit
end
write memory

What do these commands do?

enable enters privileged EXEC mode, which gives you admin access to the router.
configure terminal enters global configuration mode where you can change settings.
interface GigabitEthernet0/0 selects the router port that faces the Sales network. ip address 192.168.1.1 255.255.255.0 assigns it the gateway address for the Sales subnet.
no shutdown activates the interface. Router interfaces are off by default, so this command is required on every interface you want to use.
The same pattern repeats for GigabitEthernet0/1, which faces the Engineering network on 192.168.2.0/24.
write memory saves the configuration so it survives a reboot.

After you enter no shutdown on each interface, the link lights on the router and switches should turn green.

Verify the router interfaces are up by running this command from the router CLI:

show ip interface brief

You should see a table with columns for Interface, IP-Address, Status, and Protocol. Both GigabitEthernet0/0 and GigabitEthernet0/1 should show their assigned IP addresses with Status and Protocol both reading up.

Seeing 'administratively down' on an interface?

This means the no shutdown command was not applied to that interface. Re-enter configuration mode and run no shutdown on the affected interface.
Make sure you typed no shutdown (two words, no hyphen) exactly.

Why is my router interface still showing down?

Set default gateways on all PCs and test cross-subnet connectivity

The router is ready, but the PCs don't know it exists yet. Each PC needs a default gateway to tell it where to send traffic that's outside its own subnet.

What is a default gateway?

A default gateway is the IP address of the router interface on a device's local subnet. When a PC wants to reach an address outside its own network, it forwards the packet to the gateway, which then routes it to the correct destination.

Without a gateway, a PC can only communicate with devices on its own subnet.

Click on PC0, go to Desktop > IP Configuration.
Set the Default Gateway to 192.168.1.1.
Click on PC1, go to Desktop > IP Configuration.
Set the Default Gateway to 192.168.1.1.

Confirm the Sales gateway is reachable by opening Command Prompt on PC0 and running:

ping 192.168.1.1

You should see replies from 192.168.1.1. This proves PC0 can reach the router.

Now set the gateway for the Engineering PCs. Click on PC2, go to Desktop > IP Configuration.
Set the Default Gateway to 192.168.2.1.
Click on PC3, go to Desktop > IP Configuration.
Set the Default Gateway to 192.168.2.1.

Every PC now knows where to send traffic that's outside its own subnet. Time for the moment of truth.

Before you run the next command, predict: will PC0 (Sales, 192.168.1.10) be able to reach PC2 (Engineering, 192.168.2.10) on a completely different subnet?

Open Command Prompt on PC0 and test cross-subnet connectivity by running:

ping 192.168.2.10

You should see replies from 192.168.2.10 with 0% packet loss. The router forwarded the packet from the Sales subnet to the Engineering subnet and back again.

What just happened?

PC0 sent a ping to 192.168.2.10, which is outside the 192.168.1.0/24 subnet. PC0 checked its default gateway and forwarded the packet to 192.168.1.1 (the router).

The router looked at its routing table, saw that 192.168.2.0/24 is connected to GigabitEthernet0/1, and forwarded the packet out that interface to PC2. The reply followed the same path in reverse.

Ping to 192.168.2.10 failing?

Run show ip interface brief on the router and confirm both interfaces show up for Status and Protocol.
Check that PC0's default gateway is 192.168.1.1 and PC2's default gateway is 192.168.2.1. A missing or wrong gateway is the most common cause of cross-subnet ping failure.
Verify that PC2's IP address is 192.168.2.10 with subnet mask 255.255.255.0.

Why is my cross-subnet ping failing between Sales and Engineering?

✔️ Awesome, I've got everything!

Great work. Make sure all four PCs have the correct IP addresses and gateways set before moving on.

ⓧ I'd like to double check my configuration

Here is the expected state for every device. Compare each one against your setup.

PC0: IP 192.168.1.10, Subnet Mask 255.255.255.0, Default Gateway 192.168.1.1.

PC1: IP 192.168.1.11, Subnet Mask 255.255.255.0, Default Gateway 192.168.1.1.

PC2: IP 192.168.2.10, Subnet Mask 255.255.255.0, Default Gateway 192.168.2.1.

PC3: IP 192.168.2.11, Subnet Mask 255.255.255.0, Default Gateway 192.168.2.1.

Router: Run show ip interface brief from the router CLI. Confirm:

GigabitEthernet0/0 shows IP 192.168.1.1, Status up, Protocol up.
GigabitEthernet0/1 shows IP 192.168.2.1, Status up, Protocol up.

Cabling: Switch0 is connected to the router's GigabitEthernet0/0. Switch1 is connected to the router's GigabitEthernet0/1. All link lights are green.

Two departments, two subnets, one router connecting them. Next up, you'll ditch the manual IP configuration and set up DHCP so the router assigns addresses automatically.

Replace Manual IPs with DHCP

Your Sales and Engineering departments can now talk to each other through the router. Every PC has a static IP address you typed in by hand.

Four PCs is manageable. An office with hundreds of devices is not. DHCP automates IP address assignment so devices configure themselves the moment they connect to the network. It is also a core CompTIA Network+ topic, falling under the Network Operations domain (19% of the N10-009 exam).

In this step, get ready to:

Configure DHCP pools on the router for both department subnets.
Switch all four PCs from static IP addresses to DHCP.
Verify DHCP assignments and test cross-subnet connectivity.

Configure DHCP pools on the router

Right now, every PC has an IP address because you manually typed it in. A DHCP server removes that manual work by handing out addresses automatically when a device connects.

What is DHCP?

DHCP stands for Dynamic Host Configuration Protocol. When a device connects to the network, it broadcasts a request asking for an IP address. A DHCP server responds with an available address, a subnet mask, a default gateway, and a DNS server.

In this project, the router itself acts as the DHCP server. Each subnet gets its own DHCP pool, which is a named block of settings that tells the router which addresses to hand out and which network details to include.

Before creating the pools, you need to tell the router which addresses to keep for itself. The router's own interface addresses (192.168.1.1 and 192.168.2.1) plus a small range around them should never be handed out to PCs.

Click on Router0 in your workspace and go to the CLI tab.
Set up excluded addresses and the Sales DHCP pool by entering these commands:

enable
configure terminal
ip dhcp excluded-address 192.168.1.1 192.168.1.10
ip dhcp excluded-address 192.168.2.1 192.168.2.10
ip dhcp pool SALES
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
dns-server 8.8.8.8
exit

What do these commands do?

ip dhcp excluded-address reserves a range of addresses so the router never hands them out. You exclude .1 through .10 on both subnets to protect the router's own addresses and leave room for future static devices like printers.
ip dhcp pool SALES creates a named pool. Everything inside the pool block defines what addresses and settings PCs on that subnet receive.
default-router tells each PC where to send traffic destined for other subnets. dns-server tells each PC where to look up domain names.

You should still be in global configuration mode. Create the Engineering DHCP pool and save the configuration by entering these commands:

ip dhcp pool ENGINEERING
network 192.168.2.0 255.255.255.0
default-router 192.168.2.1
dns-server 8.8.8.8
exit
end
write memory

What does this code do?

This creates a second DHCP pool called ENGINEERING for the 192.168.2.0/24 subnet. The end command exits configuration mode, and write memory saves everything to the router's startup configuration so it survives a reboot.

After write memory runs, you should see [OK] confirming the configuration is saved.
Confirm both pools exist by running this command on the router CLI:

show ip dhcp pool

You should see two pools listed: SALES and ENGINEERING. Both will show zero leased addresses because no PC has requested an address yet.

Not seeing both pools?

Make sure you typed exit after each pool's settings. Without exit, the Engineering pool's settings may have been added inside the Sales pool.
Check that you were in global configuration mode (the prompt shows Router(config)#) when entering each ip dhcp pool command.

Help me troubleshoot my DHCP pool configuration on the router.

Switch all PCs from static to DHCP

The router is ready to hand out addresses. Now each PC needs to ask for one instead of using the static IP you configured earlier.

Click on PC0 in your workspace.
Go to the Desktop tab and select IP Configuration.
Click the DHCP radio button (it is currently set to Static).

After a moment, the IP address, subnet mask, default gateway, and DNS server fields populate automatically. The IP address will be somewhere in the 192.168.1.11+ range because you excluded .1 through .10.

Why did the fields fill in by themselves?

When you clicked DHCP, the PC sent a broadcast message on the network asking for an address. The router received that request, checked the SALES pool, picked the first available address after the excluded range, and sent it back along with the subnet mask, default gateway, and DNS server. All of that happened in the background.

Repeat the same process for PC1: click on it, go to Desktop > IP Configuration, and click the DHCP radio button.
Do the same for PC2 and PC3 in the Engineering department.
Confirm each PC's IP address field populated after clicking DHCP. Sales PCs should receive addresses in the 192.168.1.11+ range. Engineering PCs should receive addresses in the 192.168.2.11+ range.

PC stuck on 0.0.0.0?

Check that the cable connecting the PC's switch to the router shows green link lights on both ends. A red or amber light means the link is not up.
Open the router CLI and run show ip interface brief. Both GigabitEthernet interfaces should show up in both the Status and Protocol columns.
Try clicking the Static radio button and then clicking DHCP again to re-trigger the DHCP request.

Help me figure out why my PC is not getting a DHCP address.

Verify DHCP assignments and test connectivity

All four PCs should now have DHCP-assigned addresses. Time to check the router's records and make sure cross-subnet communication still works.

Click on Router0 and go to the CLI tab.

Before you run the next command, how many entries do you expect to see in the DHCP binding table?

Check the router's DHCP lease table by running this command:

show ip dhcp binding

You should see four entries, one for each PC. Each entry shows the IP address the router assigned and the hardware (MAC) address of the PC that received it. Sales PCs appear in the 192.168.1.11+ range. Engineering PCs appear in the 192.168.2.11+ range.

What is a DHCP binding?

A binding is the association between a leased IP address and the device that received it. The router keeps track of every address it hands out so it does not give the same address to two different devices.

The binding table proves the router handed out addresses. Now test whether those addresses can actually reach across subnets. Do you think the ping will still work with DHCP-assigned addresses instead of the static ones?

Click on PC0, go to Desktop, and open Command Prompt.
First, check the DHCP-assigned IP address of an Engineering PC. Click on PC2, go to Desktop > IP Configuration, and note the IP address shown.
Return to PC0's Command Prompt and ping PC2's DHCP-assigned address by running:

ping 192.168.2.11

Why 192.168.2.11?

Because the excluded range ends at .10, the first address the router hands out is .11. If PC2's IP Configuration shows a different address, use that address instead.

You should see replies from the Engineering PC with 0% packet loss. Cross-subnet routing works the same whether the addresses came from DHCP or were typed in manually.

Ping failing across subnets?

Verify that each PC's Default Gateway field was filled in by DHCP. It should show 192.168.1.1 for Sales PCs and 192.168.2.1 for Engineering PCs.
If the gateway field is blank, the DHCP pool may be missing the default-router setting. Re-enter the pool on the router CLI and add it.
The first ping packet may time out due to ARP resolution. This is normal. Look at the remaining packets for replies.

Help me debug why my cross-subnet ping is failing after switching to DHCP.

Your office network now manages itself. Every PC gets its address, gateway, and DNS server automatically the moment it connects. In the next step, you will deliberately break this network and use the Network+ troubleshooting methodology to diagnose and fix it.

Break the Network and Fix It

Your DHCP setup works perfectly. Every PC gets an address automatically, and cross-subnet pings succeed without a hitch.

But what happens when something breaks? Network Troubleshooting is the single biggest domain on the CompTIA Network+ N10-009 exam, worth 24% of your score. In this step, you will deliberately introduce a misconfiguration, watch connectivity fail, and apply the CompTIA troubleshooting methodology to diagnose and fix it.

In this step, get ready to:

Break the network by changing the router's subnet mask to the wrong value.
Diagnose the failure using the CompTIA troubleshooting methodology and Cisco IOS show commands.
Fix the misconfiguration and verify full connectivity is restored.

Introduce a deliberate misconfiguration

A subnet mask tells the router which IP addresses belong to a particular network. Changing it to the wrong value is one of the most common real-world misconfigurations, and it can silently break connectivity without taking any interfaces offline.

Click on Router0 in your workspace.
Go to the CLI tab.
Change the Sales interface's subnet mask to the wrong value by entering these commands:

enable
configure terminal
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.128
end

What do these commands do?

You changed the subnet mask on GigabitEthernet0/0 from 255.255.255.0 (/24) to 255.255.255.128 (/25). This tells the router that the Sales network only includes addresses 192.168.1.1 through 192.168.1.127 instead of through 192.168.1.255.

The interface itself stays up. There is no error message. This is exactly what makes subnet mask issues so tricky in real networks.

Observe the failure and diagnose the root cause

The CompTIA troubleshooting methodology gives you a structured sequence for diagnosing network problems. You are about to walk through the first three steps: identify the problem, establish a theory, and test the theory.

Before you test, think about what you just changed. Do you think a ping from Sales to Engineering will succeed or fail after the subnet mask change?

Click on PC0 in your workspace.
Go to the Desktop tab and open Command Prompt.
Test cross-subnet connectivity by running this command:

ping 192.168.2.10

The ping fails. You see "Request timed out" messages instead of replies. That is step one of the troubleshooting methodology: you have identified the problem.

What is the Network+ troubleshooting methodology?

The CompTIA troubleshooting methodology is a structured process tested on the N10-009 exam. You are working through the first three steps right now.

Identify the problem: cross-subnet pings fail. PCs in Sales cannot reach PCs in Engineering.
Establish a theory of probable cause: the router interface configuration may have changed.
Test the theory: run diagnostic commands on the router to compare the current configuration against what you expect.

Time for step three. You need to check the router to test your theory.

Click on Router0 and go to the CLI tab.
Check the interface status by running this command:

show ip interface brief

Both interfaces show as "up" and "up." This eliminates physical or link-layer issues, but it does not tell you about the subnet mask.

Compare the actual configuration against what you expect by running this command:

show running-config | section interface

Look at the GigabitEthernet0/0 section. You see subnet mask 255.255.255.128 instead of 255.255.255.0. That is your root cause.

Why does the wrong mask break connectivity?

The /25 mask tells the router that only addresses 192.168.1.1 through 192.168.1.127 are on the Sales network. The DHCP pool still assigns addresses from the full /24 range.

This mismatch between the router's view of the network and the actual addresses in use causes the router to mishandle traffic. The subnet mask changed, but the interfaces stayed up, so nothing obvious flagged the problem.

Fix the misconfiguration and verify

You found the root cause. The GigabitEthernet0/0 interface has the wrong subnet mask. Now correct it and save the configuration so it survives a reboot.

On the Router0 CLI tab, correct the subnet mask by entering these commands:

enable
configure terminal
interface GigabitEthernet0/0
ip address 192.168.1.1 255.255.255.0
end
write memory

What do these commands do?

ip address 192.168.1.1 255.255.255.0 restores the correct /24 subnet mask on the Sales interface.
write memory saves the running configuration to startup so the fix persists if the router restarts.

You have corrected the mask. Before you test, what do you expect to see when you ping from Sales to Engineering now?

Click on PC0 in your workspace.
Go to the Desktop tab and open Command Prompt.
Verify cross-subnet connectivity is restored by running this command:

ping 192.168.2.10

You see "Reply from 192.168.2.10" confirming the fix worked. Cross-subnet connectivity is fully restored.

Click on Router0 and go to the CLI tab.
Confirm both interfaces are healthy by running this command:

show ip interface brief

Both GigabitEthernet0/0 and GigabitEthernet0/1 show "up" status with the correct IP addresses. Your network is back to full health.

Ping still failing after the fix?

Run show running-config | section interface on the router and confirm GigabitEthernet0/0 shows mask 255.255.255.0 (not 255.255.255.128).
Check that the PCs are still set to DHCP mode. Click on each PC, go to Desktop > IP Configuration, and confirm the DHCP radio button is selected.
The first ping may time out due to ARP resolution. Try running the ping command a second time.

Still stuck? Help me troubleshoot why my cross-subnet ping is failing after I corrected the subnet mask on my router.

You just completed the full troubleshooting cycle: you broke a network, diagnosed the root cause with Cisco IOS show commands, fixed the misconfiguration, and proved the fix worked. That is real Network+ troubleshooting in action.

Secret mission

Add a Third Department

A new Marketing department needs to join the network. Extend your topology with a third switch, two PCs, a new router interface, and a DHCP pool. This time, you choose the IP addressing and write the CLI commands yourself.

Clean Up Your Resources

This project runs entirely on your local computer using Cisco Packet Tracer. There are no cloud resources or ongoing costs.

Resources you used:

Your Cisco Packet Tracer topology file (.pkt).
Cisco Packet Tracer application.

Keep everything running

No action needed. Choose this if you are still actively building or want to keep your network topology for future practice.

Your .pkt file stays on your computer. Reopen it anytime in Packet Tracer to practice or extend the network.
Your Cisco Networking Academy account remains active for future courses and Packet Tracer updates.

Pause - I'll come back to this later

Close Packet Tracer to free up memory on your computer. Your saved files stay in place for when you return.

Close Cisco Packet Tracer if it is still open.
Your .pkt file remains saved on your computer. Reopen it when you are ready to continue.

Delete - I don't want to use this again

Remove your project file and optionally uninstall Packet Tracer from your computer.

If you saved your topology, locate the .pkt file on your computer and delete it.
If you did not save, simply close Packet Tracer and click Don't Save when prompted.
To uninstall Cisco Packet Tracer, open Settings on your computer.
Select Apps.
Find Cisco Packet Tracer in the list of installed apps.
Select Uninstall and follow the prompts to remove it.

Nice Work!

You just built a multi-department office network from scratch in Cisco Packet Tracer. From placing your first router to diagnosing a broken subnet, you have covered skills that map directly to three of the five CompTIA Network+ N10-009 exam domains.

You've learned how to:

Build a routed office network with two departments on separate subnets, connected through a Cisco 2911 router using the IOS CLI to configure interfaces, IP addresses, and cross-subnet routing.
Automate IP address assignment with DHCP by creating DHCP pools on the router, excluding reserved addresses, and switching all PCs from static to dynamic configuration.
Diagnose and fix a network failure by deliberately breaking the network with a wrong subnet mask, then applying the CompTIA Network+ troubleshooting methodology to identify, diagnose, and resolve the issue using show ip interface brief and show running-config.
Secret Mission: Extended the network to three departments by adding a Marketing subnet, configuring a third router interface, and creating an additional DHCP pool.

Ready to quiz yourself?