Assume a website infrastructure is very complicated and is fully distributed (probably like most large web companies).

Am I right in thinking that although there are all these extra web servers to handle multiple client requests, there is still a single "machine" whereby users must enter? I am guessing this machine will be the one physically associated to the IP address?

I ask because I need to know whether, in places where distributed systems exist, there is still a single point of failure- usually the control node or, in this example, the machine connected to the public internet? Surely there cannot be two machines connected to the internet, as they would have to have different IP addresses?

This "machine" may not be a server per se, but maybe it is a piece of cisco equipment. I just need to know whether, in the real world, these distributed systems still have a particular section where they depend on the integrity of one electronic device?