TTL (IP)
Time To Live — 8-bit field in the IP header (IPv6 calls it "Hop Limit") that's decremented at every router hop. Reaches 0 → packet is dropped + ICMP "Time Exceeded" goes back to the sender. Anti-routing-loop mechanism.
Default start value varies per OS: Linux 64, Windows 128, Solaris 255. Usage beyond dropping loops: traceroute exploits TTL — send packet with TTL=1, 2, 3... each hop drops and the ICMP response reveals each router. "Time-to-live" is a confusing name — it's "hops to live", not seconds. Multicast TTL limits broadcast range. Modern use for OS fingerprinting: TTL = 64 ⇒ probably Linux/Mac; 128 ⇒ Windows. CDN debug: dig +trace uses DNS TTL (different concept!) for caching.