Turning Point Academy · Network Essentials

Network Troubleshooting Flowchart

The method (follow the arrows)

problem reported
  → 1. GATHER  who? since when? error text? scope: one user or many?
  → 2. ISOLATE what still works vs what does not (the boundary is the clue)
  → 3. WHAT CHANGED?  update, new cable, new config, storm last night?
  → 4. PROBABLE CAUSE — test the cheapest one first
        └─ out of your authority or depth? → ESCALATE (hand over notes)
  → 5. FIX ONE THING (plan the rollback before you touch anything)
  → 6. VERIFY — did the original complaint actually go away?
        └─ no → undo, back to 4 with the next cause
  → 7. DOCUMENT symptom, cause, fix — tomorrow's you will search for it

The bottom-up OSI ladder

Start at the floor — the cheap fixes live there. Each rung: run the check; if it passes, climb.

Physical — link lights on? cable seated, undamaged, right port, PoE budget OK? No light → this rung is your problem.
Link — interface up, speed/duplex sane? Win: Get-NetAdapter · Linux: ip link.
Ping yourself — Win: ipconfig /all · Linux: ip addr. Valid address? Then ping 127.0.0.1 and your own IP. 169.254.x.x → stop, DHCP never answered.
Ping the gateway — ping <gateway> (find it in ipconfig / ip route). Fails → local network or gateway problem, not "the Internet".
Ping a remote IP — ping 1.1.1.1; if it dies mid-path, tracert / traceroute (or mtr) shows where.
Test DNS — nslookup example.com / dig example.com. IP ping works but names fail → it was DNS.
Test the port/service — Win: Test-NetConnection host -Port 443 (with -Port it tests TCP only and does not ping) · Linux: nc -zv host 443.
Application — the network is clean; look at config, credentials, logs.

Symptom → most probable cause

Symptom	Look at
Address is 169.254.x.x (APIPA)	DHCP unreachable: server down, wrong VLAN, dead cable
Ping own IP OK, gateway fails	wrong gateway/mask setting, VLAN membership, or the gateway itself
Ping 1.1.1.1 OK, names fail	DNS — wrong server configured, or the server is down
Intermittent Wi-Fi	interference, weak coverage, channel congestion, roaming between APs
One whole VLAN dark	trunk not carrying that VLAN, its gateway (SVI) down, no DHCP scope
"Duplicate IP" warnings	a static address inside the DHCP scope, or two statics colliding
Same-site hosts: some reachable, some not	a wrong subnet mask splitting one network in two
Port dead after a device swap	port security shut it down (unknown MAC violation)
Whole segment crawling, switch LEDs solid	broadcast storm — a loop without STP, or a jabbering NIC
"Connected, no Internet", browser redirected	captive portal waiting to be accepted

Escalate when: the fix is outside your authority (ISP circuit, another team's server) · you are out of depth or the clock/SLA is burning · the next test risks damage. Hand over everything you gathered, tried and ruled out — that work is never wasted. Escalation is a skill, not a failure.

Network Essentials · Turning Point Academy — backbone: Al-Doori, Network Essentials, Ch. 12 & 2.