Time to Live
Name servers can’t cache data forever, of course. If they did, changes to that data on the authoritative name servers would never reach the rest of the network; remote name servers would just continue to use cached data. Consequently, the administrator of the zone that contains the data decides on a time to live (TTL) for the data. The time to live is the amount of time that any name server is allowed to cache the data. After the time to live expires, the name server must discard the cached data and get new data from the authoritative name servers. This also applies to negatively cached data: a name server must time out a negative answer after a period in case new data has been added on the authoritative name servers.
Deciding on a time to live for your data is essentially deciding on a trade-off between performance and consistency. A small TTL will help ensure that data in your zones is consistent across the network, because remote name servers will time it out more quickly and be forced to query your authoritative name servers more often for new data. On the other hand, this will increase the load on your name servers and lengthen the average resolution time for information in your zones.
A large TTL reduces the average time it takes to resolve information in your zones because the data can be cached longer. The drawback is that your information will be inconsistent longer if you make changes to the data on your name servers.
But enough of this theory—I’ll bet you’re ...
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