Stage 8 · Lesson 8 of 9
Network checks: DNS, ping and ports
When something "cannot connect", check one layer at a time. First, does the name turn into an address? That is , the internet's phone book, and dig example.com or nslookup answers it.
Second, run a reachability check: can you reach the machine at all? Try ping, though many servers block it. Third, is a program listening on the , the numbered door a service uses?
On the server, ss -tlnp on Linux or lsof -i :3000 shows which owns a port. That also solves "address already in use". Fourth, is a blocking you?
Check ufw status and your 's security rules. Finally, curl -v tests the service itself.
Common trapguessing and changing several things at once instead of testing one layer at a time.
A question to check yourself, an AI practice session and 2 cheat sheet entries come when you add the roadmap to your account. Add it
Resources for this stage
Start with· follow one of these
- GuideFreeInitial Server Setup with Ubuntu (DigitalOcean)
DigitalOcean · Beginner
A clear, widely used checklist for a new user, sudo, SSH access and a firewall on Ubuntu
- CourseFreeThe Missing Semester of Your CS Education
MIT · Beginner
The command-line environment lecture covers SSH, tmux, job control and dotfiles
Practise
Keep handy
Also worth your time
- roadmap.sh: LinuxGo deeper
The server, networking and troubleshooting branches of the Linux map