Network diagnostics encompass a range of protocols and techniques used to measure connectivity, latency, packet loss, and routing between systems on the internet. The Internet Control Message Protocol (ICMP, RFC 792) provides the foundation for tools like ping and traceroute, using TTL (Time to Live) field manipulation to map network paths. Each router along a path decrements the TTL by 1; when TTL reaches 0, the router sends an ICMP Time Exceeded message back to the source, revealing its IP address and response time.

BGP (Border Gateway Protocol, RFC 4271) is the inter-domain routing protocol that connects autonomous systems (AS) across the internet. It determines the best paths for routing IP packets between networks based on AS path attributes, local preference, MED, and communities. Understanding BGP routing is essential for diagnosing connectivity issues, identifying route hijacks (as seen in the Pakistan Telecom/YouTube incident of 2008 and the Rostelecom incident of 2020), and verifying RPKI Route Origin Authorization (ROA) records. Port scanning with tools like nmap reveals which TCP/UDP services are accepting connections, helping administrators verify firewall rules and identify unauthorized open ports. Network IP geolocation and ASN lookup tools help correlate IP addresses with their geographic origin and network operator, which is invaluable for threat intelligence and abuse reporting.

Command-Line Usage

Diagnose network connectivity and routing from your terminal:


macOS / Linux: ping -c 4 example.com — test basic connectivity with 4 packets
macOS: traceroute example.com — trace network path (uses UDP by default)
Linux: traceroute -T -p 443 example.com — TCP traceroute on port 443
Windows CMD: tracert example.com — trace route on Windows
Windows PowerShell: Test-NetConnection -ComputerName example.com -Port 443
Linux: nmap -sV -p 80,443,22,25 example.com — scan common ports

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