Hops and TTL
Every packet crosses the internet one router at a time; each router is a hop. This explains what TTL is, how traceroute uses it to find every hop, and how to read the result in Netutilus for Mac.
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Traffic doesn’t travel to a server directly. It’s passed from router to router, each one sending it closer: your router, your provider’s routers, the networks between, and finally the server’s. Each step is a hop. A typical route has 8 to 20.
TTL
Every packet carries a number called TTL, time to live. Each router that passes it on subtracts one. When it reaches zero, the router drops the packet and sends back a short “time exceeded” message. That keeps a packet caught in a loop from circling forever.
How traceroute uses it
Traceroute sends packets with a TTL of 1, then 2, then 3, and so on. The first one dies at the first router, which answers; the second at the second router; and so on, until a packet reaches the destination itself. Each answer reveals one hop, and how long the round trip to it took.
What that explains
- Hops that never answer. Some routers pass traffic on but never send “time exceeded”. Their row shows
*, and the route continues past them. - A slow hop with fast hops after it. Answering probes is low on a router’s list. Only delay that lasts to the destination is real.
- Several addresses for one hop. Large networks spread traffic over parallel links, so different probes meet different routers.
- Max hops. How far traceroute looks before giving up: 30 is plenty for almost any route.
Related
- Traceroute, MTR and Visual Traceroute.
- Latency.
Run it on your own network
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