Network Troubleshooting

Command-Line Diagnostics (Objective 5.7)

ping — basic reachability

• ping 127.0.0.1 tests the local TCP/IP stack (loopback); failure means a corrupt stack or driver.
• ping <gateway> confirms the local LAN path; ping 8.8.8.8 confirms Internet routing by IP.
• Request timed out = no reply (host down or ICMP blocked); Destination host unreachable = no route, a local-network problem.

tracert / traceroute

Lists each hop (router) to a destination with round-trip times, so you can see where the path breaks. Repeated * * * at one hop flags that router as the failure point or an ICMP-blocking device. traceroute is the Linux/macOS spelling.

ipconfig / ifconfig / ip addr

• ipconfig shows IP, subnet mask, default gateway.
• ipconfig /all adds the MAC address, DHCP server, DNS servers, and lease times.
• ipconfig /release then /renew requests a fresh DHCP lease; /flushdns clears the resolver cache.
• Linux/macOS use ifconfig or the modern ip addr.

nslookup, netstat, pathping

• nslookup example.com resolves a name; nslookup example.com 8.8.8.8 forces a specific DNS server to isolate DNS faults.
• netstat -an lists connections/listening ports; netstat -b (Windows) names the owning process — useful for spotting malware beacons.
• pathping combines route discovery with per-hop packet-loss statistics.

The APIPA / Gateway / DNS Decision Tree

This tree is the heart of Domain 5 networking questions: an APIPA address is never a DNS problem, and name-only failures are never cabling problems.

Wired Problem Patterns

Wireless-Specific Troubleshooting

2.4 GHz Channel Planning

The 2.4 GHz band has 11 usable U.S. channels but only three non-overlapping ones: 1, 6, and 11. Neighboring APs should be assigned to different ones of these three to avoid co-channel interference. The 5 GHz band offers many non-overlapping channels and far less congestion but shorter range. Common 2.4 GHz interferers: microwave ovens, cordless phones, Bluetooth, baby monitors, and overlapping neighbor Wi-Fi. Physical attenuators include thick walls, metal, and water.

A Structured Diagnostic Walkthrough

When a user reports "no Internet," resist random reboots and walk the path outward. First confirm the physical layer: is the Ethernet cable seated and the link light lit, or is Wi-Fi enabled and associated? Next read the IP configuration with ipconfig /all. A 169.254.x.x address means DHCP never answered — chase the cable, switch port, or DHCP scope. A valid private address (for example 192.168.1.50) means addressing is fine; move up the stack. Then ping the default gateway: success proves the local LAN works, failure points to cabling or the switch. Then ping 8.8.8.8 to test Internet routing by IP.

Finally resolve a name with nslookup or by pinging a hostname; if the IP ping succeeds but the name fails, you have proven DNS is the only broken layer and can fix the DNS server entry and run ipconfig /flushdns. This bottom-up sequence — physical, addressing, gateway, Internet, DNS — turns a vague complaint into a precise fault location and mirrors how Domain-5 scenario questions are written.

Reading the Symptoms Examiners Use

• Intermittent drops on a wired PC almost always start at the physical layer: a kinked cable, a loose RJ45 boot, or a run exceeding the 100-meter copper Ethernet limit. Check the link light before software.
• An IP conflict ("another device on the network has the same IP") knocks two hosts offline together; resolve it by moving both to DHCP or assigning unique static addresses.
• Slow throughput with lots of errors suggests a duplex/speed mismatch — set both ends to auto-negotiate rather than forcing 100/full on one side only.
• High latency that climbs at one specific hop in tracert/pathping localizes congestion or a failing router on the path; the hops before it are healthy.
• Connected but no Internet on Wi-Fi can be a captive portal waiting for a login, an AP whose own uplink is down, or wrong DNS pushed by the AP.

Knowing which tool proves which layer is the real skill: ping 127.0.0.1 validates the local TCP/IP stack, ping <gateway> validates the LAN, ping 8.8.8.8 validates routing, and nslookup validates name resolution — each isolates one variable, exactly as the troubleshooting methodology demands.