Chapter 8

Troubleshooting Connectivity

ping, mtr, firewall vs routing vs DNS.

Learning objectives

Follow a layered troubleshooting checklist
Distinguish DNS, routing, firewall, and physical issues
Use ping, mtr, and arp effectively

Layered checklist

When “the network is down,” start at the bottom — physical — and move up. Skipping layers wastes hours blaming DNS when a cable was unplugged.

Physical — link lights, Wi‑Fi associated, correct VLAN port
IP config — address, mask, gateway, DNS on device
Local gateway — ping 192.168.10.1
DNS — resolve www.workshopco.ca
Remote host — ping/traceroute to 203.0.113.10
Application — HTTPS port 443, web server up (Book 2)

Symptom matrix

Symptom	Likely layer	Next step
Cannot ping gateway	Local L2/L3	Check cable, VLAN, IP conflict
Gateway OK, DNS fails	DNS	`dig @192.168.10.1 workshopco.ca`
DNS OK, HTTPS fails	App / firewall	`curl -v https://...`, check outbound 443
Intermittent drops	Wi‑Fi / duplex	mtr over 50 cycles, check AP placement

Worked example — POS cannot reach payment API

Shop tablet pings gateway OK. DNS resolves. HTTPS to payment provider times out. Traceroute dies at ISP hop 3. Conclusion: not Workshop Co. LAN — open ticket with ISP. Meanwhile, enable offline payment mode per vendor docs.

If traceroute reaches the provider but TCP 443 fails, check firewall outbound rules on VLAN 20 — did someone block non-HTTP ports?

Ping is not everything

Some hosts block ICMP echo. Absence of ping does not prove the host is down. Test the actual service port (443, 22) with curl or nc.

Try it yourself — triage script

Write five commands (with expected success output) you would run from an office Mac when “workshopco.ca won’t load.”

Answer

ping -c 3 192.168.10.1
dig www.workshopco.ca +short
ping -c 3 203.0.113.10
curl -I https://www.workshopco.ca
traceroute 203.0.113.10

Interpret where the chain breaks.

IP conflict scenario

Printer and laptop both claim 192.168.10.40. Describe two symptoms staff might report and how to find the duplicate.

Answer

Symptoms: print jobs fail randomly; laptop drops off Wi‑Fi. Check router DHCP log, ping .40 and compare ARP MAC addresses, verify reservation matches printer MAC only.

Quick quiz

What does a successful gateway ping prove?
DNS works but site fails — name two non-DNS causes.
Why use mtr instead of one ping?

Answers

Local IP config and L2 path to router are likely OK — not end-to-end internet.
Web server down, TLS misconfig, firewall blocking 443, wrong IP in DNS.
Continuous stats per hop — spot packet loss/latency patterns over time.