Network+ exam prep: network troubleshooting
On a network with injected faults you methodically find problems at every layer from the cable to DNS, prove the root cause and fix it for good.
Level 201 · 15 lessons · 12 practice labs
Lessons
Method and tools
- The troubleshooting method: identify, theorise, test, implement, document
- Command line tools: ping, traceroute, dig, ss, tcpdump
- Hardware tools: cable tester, OTDR, tone generator
- Short task: document a connectivity problem in seven steps
Physical and link problems
- Cable problems: attenuation, crosstalk, wrong pinout
- Speed and duplex mismatch
- Wireless problems: interference, signal strength, roaming
- Finding a client in the wrong VLAN
- Short task: fix three clients that can’t connect, one by one
Network and service problems
- Duplicate IP addresses and wrong subnet masks
- Gateway and routing loop problems
- Connectivity problems caused by DNS and DHCP
- The MTU problem where large packets disappear
- A slow network: congestion, broadcast storms, high latency
- Capstone: fix a fault-ridden network down to the root causes
Practice labs
- Find the hop where the path breaks: With traceroute and ping you find which router the path to a server breaks at and record the evidence.
- Service running but unreachable: You find from ss output that a running service listens only on localhost, make it listen on the right address and test remotely.
- Solve an unfinished handshake with tcpdump: You show with tcpdump that a client sends SYN without a reply and find and fix the firewall rule cutting the path.
- Fix a duplex mismatch: You match the speed and duplex of an interface with rising error counters to the far end and show the errors stop.
- Put a client in the wrong VLAN back: Among three clients without an address you find the one in the wrong VLAN, move it and separate out the other two problems.
- Repair a tunnel where large packets vanish: On a tunnel where small pings pass but file transfers hang you measure the MTU limit and fix it with a suitable MTU or MSS setting.
- Fix a network handed the wrong DNS server: You find DHCP handing out an old DNS server on a network where clients can't resolve names, fix the scope and renew the clients.
- Break a routing loop: You show with traceroute the loop between two routers for a destination returning TTL exceeded and fix the bad route.
- Stop a broadcast storm slowing the network: You find the storm's source from the broadcast rate in a capture, cut the looping link and measure the network returning to normal.
- Record a connectivity fault in seven steps: You log a simple connectivity fault through the identify, theorise, test, plan, implement, verify and document steps in a record file.
- Fix a branch with two faults in turn: In a branch where both the gateway and the DNS setting are broken you separate the symptoms and fix both faults in turn, verifying each step.
- Trace five random faults to their root causes: You find faults the lab broke at random on five layers and report the root cause and lasting fix for each.
About this course
On a network with injected faults you methodically find problems at every layer from the cable to DNS, prove the root cause and fix it for good.
What you will learn
- Method and tools
- Physical and link problems
- Network and service problems
Prerequisites
Finishing the level 101 courses of the same path, or knowing those topics, is enough.
Lab environment
Lessons run in your own Kali Linux terminal in the browser, connected to a private lab network. Nothing to install.
Frequently asked questions
- Is this course right for my level?
- The course is at level 201; it assumes you know the topics of the lower-level courses.
- Is there anything to install?
- No. Every lesson runs in your own lab environment that opens in the browser.
- How do lessons work?
- Each lesson has 8 to 10 steps and a short quiz. Early steps show the command to type, later steps give less help; if two attempts fail, the solution opens.
- Can I try it for free?
- Yes. The first three lessons of every course are free.