TTL as a diagnostic instrument
The time-to-live field quietly tells you how many hops a packet took. That is more useful than it sounds.
Writing
routing
Asymmetric routing and the return path that betrays you
Traffic goes out one way and comes back another. Everything works until a stateful device is involved.
Reading a routing table you did not build
Somebody else built it, nobody documented it, and it is now yours. Here is the order to read it in.