The problems are said to be connected to DNS (Domain Name Servers) hosts.
Major DNS host Dyn said it was aware of problems with its service: "Starting at 11:10 UTC on 21 October 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure.
"Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available."
Dyn then added: "This attack is mainly impacting US East and is impacting managed DNS customer in this region. Our engineers are continuing to work on mitigating this issue."
We are aware of the ongoing service interruption of our Managed DNS network. For more information visit our status page.— Dyn (@Dyn) October 21, 2016
|What is a DdoS attack?|
DdoS stands for Distributed denial of Service.
An attack occurs when an online service such as a website is bombarded by so much traffic that it crashes.
Companies can put up a shield to try and protect themselves from attacks, but many hackers can get around those protections.
If you really wanted to take down a website, it is possible (but usually illegal) to pay for sustained attacks that will hammer a site with traffic over a prolonged period.
Major websites like the ones affected today will have very high levels of security around their servers, so the attack that's brought them down must be a big one.
Meanwhile, those who could access Twitter were able to see the funny side.
Trump Plan D: have the Russians Take down half the internet so nobody can see what he said last night.— Neil Wehrle (@neilw) October 21, 2016