How the Great Firewall of Britain works

a flowchart showing how Cleanfeed — the secret British national
firewall that is presently restricting access to Wikipedia – operates:

routers firstly check the IP address of the server that’s hosting the
URL you’re trying to access. If they determine that the IP address is
also used to host one of the websites on the IWF list then your request
is passed to the IWF proxies.
A lookup is then done and if the address you’re trying to access
matches one on the list then the request is denied.

Translation: a third party now monitors every request
made to Wikipedia from the six ISPs that participate in the Great
Firewall of Britain.

Great Firewall of Britain

(Thanks, Seth!)