Troubleshooting TCP-IP Connections Conclusion

Ping

Command Syntax for Ping:

ping [destination_ip_address]

Ping Usage:

1. Run `ping` to a known-good external IP address to test basic connectivity:
   

   ping 8.8.8.8
   

2. Ping the local loopback address to confirm that the network stack on the local machine is functional:
   

   ping 127.0.0.1
   

Ping Interpretation:

1. Successful pings confirm IP-level connectivity.
2. Failure indicates network isolation or misconfiguration at the IP layer.

Netstat

Command Syntax:

netstat [options]

Netstat Usage:

1. To list all active TCP connections:

   netstat -t
   

2. To display all listening ports:

   netstat -l
   

3. To show numerical addresses instead of resolving hostnames:
   

   netstat -n
   

Netstat Interpretation:

1. Active connections and listening ports give insights into the flow of data, helping identify unexpected traffic or unresponsive services.
2. If a service should be listening but isn’t, this suggests a configuration issue or that the service isn’t running.

Address Resolution Protocol (ARP)

ARP Usage:

1. Display the ARP table:
   

   arp -a
   

2. Delete an ARP entry (useful when an incorrect association is suspected):

   arp -d [ip_address]
   

3. Add a static ARP entry:

   arp -s [ip_address] [mac_address]
   

ARP Interpretation:

1. If the ARP table populates correctly, this suggests that ARP is functioning as expected, confirming link-layer connectivity.
2. If an expected entry is missing or incorrect, this indicates an issue at the ARP or link layer, such as a disconnected cable or misconfigured switch.

Combined Troubleshooting Workflow:

1. Confirm IP Connectivity: Use `ping` to establish that the IP layer is functional.
2. Identify Active Connections and Listening Ports: Use `netstat` to gain an overview of the network state.
3. Validate Link-Layer Connectivity: Use `arp` to ensure that IP addresses are correctly mapped to MAC addresses.