What is TTL and where you will come across it

With DNS, networks and elsewhere you come across the abbreviation TTL. It means “time to live” and says how long or how far something stays valid.

What TTL is

TTL (Time To Live) is a value that sets until when or how far something is valid. You will most often meet it in two senses: with DNS and with network packets.

TTL in DNS

With DNS, TTL says how long a device may remember (cache) a record before it asks again. A low TTL means changes take effect quickly, but there are more queries. A higher TTL means fewer queries, but a change (such as moving a website to a new server) takes effect more slowly. That is exactly why TTL is lowered temporarily before moving a website.

TTL in network packets

On a network, every packet has its own TTL, which sets how many devices (hops) it can pass through before the network drops it. This prevents a packet from circling forever in the network when something goes wrong somewhere. The traceroute tool uses TTL precisely to map the path to a destination.

Where you deal with it in practice

Most often when managing a domain and DNS records, for example when you change hosting or deal with domain expiry. Then it is good to lower the TTL in advance, so the change takes effect quickly and users do not end up on the old server.

Summary

TTL is “time to live”: with DNS it sets how long a record stays valid, with packets how far they travel. In both cases it is about protection from needless load and looping.

This article is part of our Computer networks overview.