4.5 Given a scenario, troubleshoot network connectivity issues.

📘CompTIA Server+ (SK0-005)

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🔧 1. Physical Layer Checks (First Step in Troubleshooting)

Before using commands, always check basic hardware connectivity.

✅ Check Link Lights

• Network Interface Cards (NICs), switches, and routers have LED lights.
• These lights show:
  • Green/solid → Connection is active
  • Blinking → Data is being transmitted
  • No light → No connection

What it helps identify:

• Disconnected cable
• Faulty NIC
• Switch port issue

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✅ Confirm Power Supply

• Ensure all devices are powered:
  • Server
  • Switch
  • Router
  • Firewall

What to check:

• Power cables are connected
• Power supply unit (PSU) is working
• Device is turned on

Issues detected:

• Device offline
• Network path interruption

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✅ Verify Cable Integrity

• Check if cables are:
  • Damaged
  • Loose
  • Bent or broken

Common cable issues:

• Broken internal wires
• Worn-out connectors
• Loose RJ-45 connections

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✅ Check Appropriate Cable Selection

Different cables serve different purposes:

Cable Type	Use
Straight-through	PC to switch
Crossover	Switch to switch (older networks)
Fiber optic	High-speed, long-distance

Issues:

• Wrong cable type → No communication
• Incorrect port usage

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💻 2. Command-Line Tools (Most Important for Exam)

You must understand what each command does, when to use it, and key outputs.

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🌐 IP Configuration Commands

✅ ipconfig (Windows)

• Displays IP configuration

Key uses:

• Check IP address
• Verify subnet mask
• Identify default gateway

Important options:

• ipconfig /all → Detailed info
• ipconfig /release → Release IP
• ipconfig /renew → Request new IP

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✅ ip addr (Linux)

• Shows IP address configuration

What it shows:

• IP address
• Interface status (UP/DOWN)
• MAC address

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📡 Connectivity Testing Tools

✅ ping

• Tests connectivity between two devices

How it works:

• Sends ICMP Echo Requests
• Receives ICMP Echo Replies

What to look for:

• Success → Network reachable
• Failure → Possible issue

Common problems:

• Destination unreachable
• Request timed out

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✅ tracert (Windows) / traceroute (Linux)

• Tracks path packets take to destination

Shows:

• Each router (hop)
• Delay between hops

Use cases:

• Identify where connection fails
• Detect slow network segments

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🌍 DNS Troubleshooting Tools

✅ nslookup

• Queries DNS servers

Use:

• Resolve domain name to IP
• Check DNS server functionality

Output:

• IP address of domain
• DNS server used

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✅ dig (Linux)

• Advanced DNS lookup tool

Provides:

• Detailed DNS response
• Query time
• DNS record types

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📊 Network Connection Analysis

✅ netstat

• Displays network connections and statistics

Common uses:

• View active connections
• Check open ports
• Identify listening services

Important flags:

• netstat -an → All connections
• netstat -b → Process using port (Windows)

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🔌 Port and Service Testing

✅ telnet

• Tests connectivity to a specific port

Example use:

• Check if a server port (like 80 or 443) is open

Result:

• Connection success → Service reachable
• Failure → Port blocked or service down

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✅ nc (Netcat)

• Advanced networking tool

Uses:

• Port scanning
• Connectivity testing
• Data transfer testing

Key advantage:

• More powerful than telnet

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🖧 Legacy and Specialized Tools

✅ nbtstat

• Used for NetBIOS over TCP/IP

Functions:

• View NetBIOS name table
• Check name resolution

Used in:

• Older Windows networks

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✅ route

• Displays and modifies routing table

Shows:

• Network paths
• Default gateway
• Static routes

Why important:

• Incorrect routing = network unreachable

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🧠 How to Use These Tools in Troubleshooting (Exam Strategy)

Follow a logical troubleshooting order:

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Step 1: Check Physical Layer

• Link lights
• Power
• Cables

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Step 2: Check IP Configuration

• Use ipconfig / ip addr
• Verify:
  • IP address
  • Subnet mask
  • Gateway

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Step 3: Test Connectivity

• ping localhost → test local stack
• ping gateway → test local network
• ping remote IP → test external network

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Step 4: Trace the Path

• Use tracert / traceroute

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Step 5: Verify DNS

• Use nslookup or dig

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Step 6: Check Ports and Services

• Use telnet or nc

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Step 7: Analyze Network State

• Use netstat

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Step 8: Check Routing

• Use route

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⚠️ Common Exam Tips

• Ping works but website fails → DNS issue
• Cannot ping gateway → local network problem
• Link light off → physical issue
• IP starts with 169.254.x.x → DHCP failure
• Tracert fails at first hop → local problem
• Port closed → service not running or firewall blocking

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✅ Summary (Must Remember)

• Always start with physical checks
• Use ipconfig/ip addr to verify IP settings
• Use ping to test connectivity
• Use tracert/traceroute to find where failure occurs
• Use nslookup/dig for DNS issues
• Use netstat to check ports and connections
• Use telnet/nc to test services
• Use route to verify network paths