Networking fundamentals to network troubleshooting
Understand how data really moves, from addresses and subnets to DNS and TLS, then learn to find faults methodically instead of guessing.
- Who it's for
- Anyone in or heading into IT support, or anyone who has ever said "the internet is down" and wanted to know why.
- Start with
- You can use a computer and a terminal. No networking knowledge assumed.
- Pace
- Self-paced. Most milestones take an evening or two of reading and practice.
The route
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Addresses, subnets and gateways
What an IP address and subnet mask mean, why private ranges exist, and what the default gateway is for.
Practice: Find your own IP address, mask, gateway and DNS server, and work out which addresses count as local.
- Trace a request from your laptop to a websiteByteStranger lab, interactive
- RFC 1918: Address allocation for private internetsRFC Editor
How names become addresses
Resolvers, the DNS hierarchy, record types (A, AAAA, CNAME, MX) and TTLs.
Practice: Look up a domain with dig or nslookup, then ask a different resolver and compare the answers.
- What is DNS?Cloudflare Learning Center
TCP, UDP and ports
Connections, the three-way handshake, well-known ports, and why ping is not a service check.
Practice: Test port 443 on a website with nc or Test-NetConnection, then test a port you know is closed.
- Trace a request from your laptop to a websiteByteStranger lab, interactive
- What is TCP/IP?Cloudflare Learning Center
TLS and certificates
What a certificate proves, how the handshake works, and the common certificate errors.
Practice: Use curl -v against an HTTPS site and identify the certificate's subject and expiry date.
- What happens in a TLS handshake?Cloudflare Learning Center
A troubleshooting method
Working bottom-up, splitting the problem in half, and reading error messages precisely.
Practice: Solve all four faults in the connectivity drill using only command output.
- Can't reach the service: a DNS and connectivity drillByteStranger lab, interactive
Seeing the packets
Capturing traffic to confirm what is really on the wire, when commands alone are not enough.
Practice: Capture your own DNS lookup in Wireshark and find the query and the response.
- Wireshark User's GuideWireshark
Make the checks repeatable
Turning the manual method into a script you can reuse and hand to others.
Practice: Run the connectivity check script, then extend it to test a second resolver.
- Automate the checks: a reusable connectivity scriptByteStranger lab
By the end
- Explain each stage of a network connection and which command tests it
- Diagnose DNS, routing, firewall and certificate faults from evidence
- Capture and read basic traffic to confirm a theory
This path is a guide for independent learning. It isn't accredited and doesn't award a qualification. The external resources belong to their publishers. Progress you mark is saved only in this browser.