I have a single machine that I use to host multiple websites with virtual hosts (lighttpd webserver), I can't say I am in a professional environment as I am hosting mainly for small websites and projects.
Eventually, the resources of a single machine will not be enough to cover the requests amount and I will need to add another one to my network.
What is a common technique to route the requests to the right server? It is a really simple structure, visually imagine it like this:
SERVER X website A (vhost) website B (vhost) website C (vhost) SERVER Y website D (vhost) website E (vhost) website n (vhost)