How DNS caching works.

A short-term memory of DNS answers. When your device looks up a name it keeps the answer for the record's TTL (time to live), so it doesn't have to ask again for every page or email. Your browser, your operating system, your router and your internet provider's resolver each keep one.

It throws away the remembered answers, so the next lookup asks again and gets the current record. It doesn't change anything on the internet and doesn't affect anyone else, and it's always safe: the worst case is that the next few page loads take a few milliseconds longer.

After you change a DNS record or move a site to a new host and you still see the old one while others see the new one, after nameserver changes, or when a site won't load for you but loads elsewhere. If the domain's own nameservers still return the old record, flushing won't help: the change hasn't been made yet.

Another cache still has it. Browsers keep their own (clear it on chrome://net-internals/#dns), your router may cache, and your internet provider's resolver keeps the old answer until its TTL runs out. Flush Google's or Cloudflare's cache and switch your device to that resolver, or wait. Your browser may also be showing a cached copy of the page itself: try a private window.

Everyone who uses that resolver, which is a large share of people. Every other resolver, including most internet providers', keeps its copy until the TTL runs out. Nobody can clear every cache in the world; lowering the TTL a day before a planned change is the only way to make a change reach everyone quickly.

Up to the record's TTL, which is often an hour and can be a day. Windows and macOS follow the TTL, Chrome keeps answers for about a minute, and some resolvers keep answers a little longer or shorter than asked. A 'no such name' answer is cached too, for the time set in the domain's SOA record.

On a Mac and on Linux, yes: the commands start with sudo and ask for your password. On Windows, ipconfig /flushdns usually works in an ordinary Command Prompt; if it says the action needs elevation, run Command Prompt as administrator. Browser and phone steps need no special rights.

Yes. It deletes nothing you need and nothing personal, and the cache fills up again by itself as you browse.