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Networking Fundamentals · Foundational

Time to Live (TTL)

An 8-bit IPv4 header field decremented by each router; when it reaches zero the packet is discarded to prevent endless looping.

Definition

The sender sets an initial TTL, commonly 64 (Linux and macOS), 128 (Windows) or 255 (many network devices). Each router decrements it by one, and a router that decrements it to zero drops the packet and returns an ICMP Time Exceeded message. Traceroute exploits this behavior to discover each hop. IPv6 renames the field Hop Limit.

Example

A ping reply showing TTL=117 from a Windows host suggests it is 11 hops away (128 minus 117).

Exam tip

Traceroute works by sending packets with TTL 1, 2, 3 and so on; DNS also has a separate, unrelated TTL for cache lifetime.

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