Traceroute: Mapping the Network Path to a Host

How Traceroute Works

Traceroute maps every hop between the source and destination by exploiting the IP Time To Live (TTL) field defined in RFC 791. It sends a series of packets with incrementally increasing TTL values. The first packet has TTL=1 — the first router decrements it to zero, drops the packet, and sends back an ICMP Time Exceeded message (RFC 792 Type 11) revealing its address. The next packet has TTL=2, reaching the second hop before expiring. This continues until a packet reaches the final destination or the maximum hop count is reached.

The mxio Traceroute tool runs this from our infrastructure, showing you the network path from our servers to any destination. This external vantage point is useful for diagnosing routing issues that may not be visible from your own network.

Reading the Results

Each line in the output represents one hop along the path. For each hop, you see:

• Hop number — Position in the path (1 is the first router after the source)
• IP address and hostname — The router or device at that hop
• RTT values — Round-trip time for each probe sent to that hop

What the Numbers Mean

Look for patterns, not individual values. A single hop with slightly higher RTT than the one before it is normal — routers at internet exchange points or international links add latency. What matters is whether the increased latency persists through all subsequent hops. If hop 5 shows 80ms but hop 6 returns to 40ms, hop 5 is not a bottleneck — its router is just slow to generate ICMP responses. If hop 5 shows 80ms and every hop after it shows 80ms or more, the latency was introduced at hop 5.

Asterisks and Timeouts

An asterisk (*) means no response was received from that hop within the timeout period. This does not necessarily mean packets are being dropped. Many routers are configured to deprioritize or ignore ICMP Time Exceeded messages — the packets pass through the router fine, but the router does not bother reporting back. A row of asterisks followed by successful hops is common and usually harmless.

A traceroute that ends in asterisks — the destination never responds — may indicate the target host is down, blocking ICMP, or unreachable due to a routing problem.

Identifying Problem Hops

The pattern to watch for: a sudden jump in RTT that persists through all subsequent hops. That jump represents a genuine latency increase, likely at a congested link, an international crossing, or a misconfigured route. Intermittent timeouts at a single hop without affecting downstream hops are almost always cosmetic — the router handles transit traffic fine but rate-limits ICMP responses.

When to Use Traceroute

Start with ping to confirm whether a host is reachable and measure overall latency. If ping shows high latency or packet loss, run traceroute to isolate where in the path the problem occurs. For DNS resolution issues, verify the domain resolves correctly with an A Lookup before testing the network path.

Run a traceroute to any host