# Troubleshooting Scenarios Practice ## Scenario 1: No Internet Access on Single PC Problem: One PC cannot access the internet. All other PCs on the same network work fine. Step 1: Check physical connection. Is the Ethernet cable plugged in or is WiFi connected. Look for link lights on the NIC. Step 2: Check IP configuration. Run ipconfig on Windows or ip addr on Linux. Verify the IP address is in the correct range. If the address starts with 169.254 the device has an APIPA address meaning DHCP failed. Step 3: Ping the default gateway. If this fails the problem is between the PC and the switch or router. Check the cable and port on the switch. Step 4: Ping a known internet IP such as 8.8.8.8. If the gateway ping worked but this fails the problem is at the router or ISP level. But since other PCs work this is unlikely. Step 5: Ping a domain name like google.com. If this fails but pinging 8.8.8.8 works the problem is DNS. Check the DNS server settings on the PC. Step 6: Check firewall settings on the PC. A software firewall might be blocking outbound traffic. Step 7: Try releasing and renewing the DHCP lease with ipconfig slash release and ipconfig slash renew on Windows. Resolution: Likely causes are incorrect manual IP configuration, DHCP failure, incorrect DNS settings, or software firewall blocking traffic. ## Scenario 2: Entire Network Cannot Access Internet Problem: All PCs on the network cannot access the internet but can communicate with each other. Step 1: Check if the router has internet connectivity. Log into the router web interface and check the WAN connection status. Step 2: Check if the router has received an IP address from the ISP. The WAN interface should have a public IP address. Step 3: Ping the ISP gateway from the router. If this fails the problem is between the router and the ISP. Step 4: Contact the ISP to verify the line is active and check for outages in the area. Step 5: Check if the router has correct DNS server settings pointing to valid public DNS servers. Step 6: Reboot the router and modem. Many connectivity issues are resolved by power cycling. Step 7: Check if the router firewall or ACLs are incorrectly blocking outbound traffic. Resolution: Likely causes are ISP outage, PPPoE credentials changed, modem failure, router WAN configuration error, or payment issue with ISP. ## Scenario 3: Can Ping IP but Cannot Open Website Problem: Users can ping 8.8.8.8 successfully but cannot open any websites in the browser. Analysis: Pinging 8.8.8.8 confirms TCP/IP networking works. Not being able to open websites while IP pings work suggests a DNS problem. Step 1: Try pinging a domain name like google.com. If this fails DNS resolution is broken. Step 2: Run nslookup google.com to check if DNS queries are being resolved. Step 3: Check the DNS server address configured on the affected devices. An incorrect DNS server address will cause all name resolution to fail. Step 4: Try manually setting the DNS server to 8.8.8.8 and test again. Step 5: If some websites work but others do not the problem might be specific to the failing DNS records or browser proxy settings. Step 6: Check if any proxy is configured in the browser or operating system network settings. Resolution: Likely causes are incorrect DNS server address, DNS server is down, or browser/system proxy misconfiguration. ## Scenario 4: Intermittent Connectivity Problem: Users experience random disconnections that resolve themselves after a few minutes. Step 1: Check error counters on switch ports. High error counts or flapping ports indicate cabling or NIC problems. Step 2: Check if the problem correlates with specific times such as peak usage hours suggesting bandwidth saturation. Step 3: Run continuous pings to the gateway and note the pattern of failures. Step 4: Check the router and switch logs for error messages around the times of failures. Step 5: Check for IP address conflicts by running arp -a and looking for duplicate IP to MAC mappings. Step 6: Check for spanning tree topology changes in switch logs which can cause temporary forwarding disruptions. Step 7: Check for DHCP lease conflicts or scope exhaustion if many devices are affected simultaneously. Resolution: Likely causes are faulty cable, duplex mismatch, IP address conflict, DHCP scope exhaustion, or spanning tree instability. ## Scenario 5: Slow File Transfer Between Two PCs Problem: File transfers between two PCs on the same local network are very slow taking minutes for a small file. Step 1: Test network speed with a network testing tool to determine actual throughput. Step 2: Check if both PC NICs and switch ports are running at the expected speed. Run at 100 Mbps or 1 Gbps not 10 Mbps. Step 3: Check for duplex mismatch. One side set to full duplex and other to half duplex causes collisions and poor performance. Step 4: Test if the problem is specific to SMB file sharing or affects all protocols. Try FTP or a direct copy and compare speeds. Step 5: Check the CPU and disk utilization on both PCs during the transfer. Slow disk or high CPU may be the bottleneck. Step 6: Check for virus or malware consuming network resources. Step 7: Check if switches between the two PCs are operating correctly and are not oversubscribed. Resolution: Likely causes are duplex mismatch, autonegotiation failure, faulty cable, switch port issue, or hardware bottleneck on one of the PCs.