Why some clients lag

A college moves its results portal to a new server. The A record for results.college.edu is changed from 10.4.1.20 to 10.4.1.55 at 09:00.

By 09:10 the network team confirms the change is live on the authoritative server.

Through the morning, students report inconsistent behaviour. Some reach the new server immediately. Others keep landing on the old one for hours. A few see it switch back and forth between reloads.

The record's TTL was set to 86400 seconds, and nobody lowered it before the change.