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Networking › Module 5 › Lesson 2

BeginnerModule 5Lesson 2/4

Network Troubleshooting Steps

A layered checklist from physical link to application

15 min+41 XP3 quiz
Module progress2 of 4

Opening

Work top to bottom—or bottom to top

Random guessing wastes time. Structured troubleshooting moves through layers: is the cable/Wi-Fi up? Do you have an IP? Can you reach the gateway? Does DNS resolve? Does the app port respond? This checklist matches how SOC and IT teams isolate faults.

1. Layer 1–2: Link and Local IP

  • Physical / Wi-Fi

    Cable seated? Wi-Fi connected? Interface UP in ip link or ip a?

  • IP configuration

    Valid address from DHCP or static? Correct subnet mask? Run ip a or ipconfig.

  • Local loopback

    ping 127.0.0.1 confirms the TCP/IP stack on the host itself works.

2. Layer 3: Gateway and Routing

Routing and gateway checksip route ping -c 3 192.168.1.1 ping -c 3 8.8.8.8

ip route
ping -c 3 192.168.1.1
ping -c 3 8.8.8.8

Ping the gateway (adjust IP for your network). Failure here points to switch, AP, or local firewall—not the internet yet. Ping a public IP by address. Success means routing works; failure may be ISP or upstream firewall.

3. Layer 4–7: DNS and Application

DNS and application testsdig cyberlium.com nslookup cyberlium.com curl -v https://cyberlium.com nc -zv cyberlium.com 443

dig cyberlium.com
nslookup cyberlium.com
curl -v https://cyberlium.com
nc -zv cyberlium.com 443

If 8.8.8.8 works but cyberlium.com fails, suspect DNS. If DNS resolves but the site fails, read curl errors: connection refused (nothing listening), timeout (filtered/blocked), TLS errors (certificate or proxy).

4. Document as You Go

Write down each test and result:

  • What workedNarrows which layer is still broken.

    Narrows which layer is still broken.

  • What failedFirst failure often points to root cause.

    First failure often points to root cause.

  • Recent changesVPN, firewall rule, or DNS change?

    VPN, firewall rule, or DNS change?

Knowledge Check

1

If ping to 8.8.8.8 works but domain names fail, you should check:

Multiple choice

Knowledge Check

2

Pinging the default gateway first helps determine if:

Multiple choice

Knowledge Check

3

curl -v is useful during troubleshooting because it:

Multiple choice

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Answer all 3 knowledge checks to continue. (0/3 answered)

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