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How to Check IP Routing?

Published in Network Diagnostics 3 mins read

To check IP routing, you can primarily use the traceroute command. This tool helps diagnose problems by showing the path a packet takes to reach its destination.

Understanding Traceroute

The traceroute command works by sending out packets with incrementally increasing time-to-live (TTL) values. Each router along the path decrements the TTL. When the TTL reaches zero, the router sends back an "ICMP time exceeded" message. By examining these messages, traceroute maps the route the packets follow.

Traceroute vs. Ping

It's important to understand how traceroute differs from the ping command:

Feature traceroute ping
Purpose Traces the route a packet takes to reach its destination, showing each hop. Checks if a host is reachable and measures round trip time.
Error Display Shows where a route fails by listing each router on the path. Returns a single error if the destination is unreachable without showing the path.
Path Details Provides a detailed view of the network path, including IP addresses and host names (if DNS resolution is available). Provides no information about the path taken by the packet.

How to Use Traceroute

  1. Basic Syntax: On most systems, the command is simply traceroute [destination IP or hostname]. For example, traceroute google.com.

  2. Interpreting Results: The output typically lists each hop (router) along the path, along with the IP address, the host name (if available), and the round-trip time for each hop.

    traceroute to google.com (142.250.179.142), 30 hops max, 60 byte packets
 1  192.168.1.1 (192.168.1.1)  1.249 ms  0.811 ms  0.722 ms
 2  10.20.30.1 (10.20.30.1)  1.495 ms  1.222 ms  1.160 ms
 3  ...
 4  ...
 5  142.250.179.142 (142.250.179.142) 10.563 ms  10.653 ms  10.225 ms

  3. Troubleshooting:

    • Hops with asterisks (*): This usually means the router did not respond to the traceroute probe. It could indicate a network problem, firewall blocking ICMP, or simply that the router is not configured to respond to ICMP messages.
    • High Latency: An unusually long response time for a particular hop can indicate congestion or other issues along the path.
    • Unreachable Destination: If the destination is not reachable, the traceroute will show that the packets stop at some point on the path and usually display a message like "!H" (host unreachable).

Practical Applications

  • Identifying network bottlenecks: Use traceroute to pinpoint where delays occur and to identify slow performing routers.
  • Verifying route changes: When routing changes, use traceroute to observe the new route.
  • Troubleshooting path failures: If a connection to a server is not working, use traceroute to see how far the traffic is reaching the target.

By using traceroute, you can efficiently check IP routing and pinpoint any issues within the network path.

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