5.3 Given a scenario, troubleshoot common issues with network services
Addressing Issues
📘CompTIA Network+ (N10-009)
1. What is a Default Gateway?
A default gateway is a network device (usually a router) that a device (like a computer or server) uses to send traffic to destinations outside its own local network.
- Think of it as a “door” from your local network to other networks.
- Every device in a network usually has an IP address, a subnet mask, and a default gateway configured.
- Without a correct default gateway, devices cannot communicate with devices outside their local subnet, even if the internet or other networks are working fine.
Example in IT terms:
- Your workstation is on subnet 192.168.1.0/24.
- Its IP is 192.168.1.10, subnet mask 255.255.255.0, and default gateway 192.168.1.1.
- If the default gateway is changed to 192.168.2.1, your computer cannot reach anything outside the 192.168.1.0 network, like the internet or a server on a different subnet.
2. How Incorrect Default Gateway Happens
Incorrect default gateway can happen in several ways:
- Manual misconfiguration – Someone typed the wrong IP address while setting up a device.
- DHCP errors – DHCP (Dynamic Host Configuration Protocol) assigns the wrong gateway IP to devices.
- Network changes – The router IP changed, but devices were not updated with the new default gateway.
- Device replacement – A new router is installed with a different IP, but devices still point to the old gateway.
3. Symptoms of an Incorrect Default Gateway
Devices with the wrong default gateway typically show the following symptoms:
- Can ping devices on the same subnet, but cannot ping devices on other subnets.
- No internet access (if the internet is outside the local subnet).
- Applications that require external network access fail to connect.
- Some network monitoring tools may report “network unreachable” errors.
4. How to Troubleshoot Incorrect Default Gateway
When troubleshooting, follow these steps:
Step 1: Check the device’s IP configuration
- Windows:
ipconfigin Command Prompt - Linux/macOS:
ifconfigorip addr - Look at:
- IP Address
- Subnet Mask
- Default Gateway
Step 2: Verify connectivity to the gateway
- Ping the gateway IP:
ping <default_gateway_IP> - If ping fails: the gateway is unreachable, or it is incorrect.
Step 3: Check network topology
- Ensure the default gateway is on the same subnet as the device.
- Example: Device IP 192.168.1.10/24 → Gateway must be 192.168.1.x.
Step 4: Correct the default gateway
- Manual configuration:
- Windows:
Control Panel → Network Settings → IPv4 → Default Gateway - Linux:
sudo ip route add default via <gateway>
- Windows:
- DHCP configuration:
- Check the DHCP server’s scope for the correct default gateway IP.
- Make sure the DHCP server is assigning the right IP.
Step 5: Test connectivity
- After correction, ping the gateway, then try an external IP like
8.8.8.8. - If successful, the issue is resolved.
5. Exam Tips
- Remember the symptoms: Local network works, but outside network/internet doesn’t.
- Default gateway must be on the same subnet as the device.
- Troubleshoot with ping first – Always verify connectivity to the gateway.
- Check DHCP vs manual settings – A wrong gateway can come from either.
- Commands to know:
- Windows:
ipconfig /all,ping,tracert - Linux/macOS:
ifconfig,ip addr,ping,traceroute
- Windows:
6. Quick Recap Table
| Topic | Key Points |
|---|---|
| Default Gateway | Device used to access networks outside the local subnet |
| Symptoms of wrong gateway | Local network works, internet fails, cannot reach other subnets |
| Causes | Manual misconfig, DHCP error, network/router changes |
| Troubleshooting Steps | Check IP config → Ping gateway → Correct gateway → Test |
| Commands to Know | ipconfig, ifconfig, ping, tracert, traceroute |
