Each machine has its own copy of everything it needs to run the  Web application which includes the HTML files, any script pages (ASP, ASP.Net), any binary files (such as compiled .Net assemblies, COM objects or DLLs loaded from the Web app) and any support files such as configuration and local data files (if any).

This means each server must be configured separately and run the Web server as well as any Web server applications that are running. If you’re running a static site, all HTML files and images must be replicated across servers

if any machine in the cluster goes down, NLB will re-balance the incoming requests to the still running servers in the cluster. The servers in the cluster need to be able to communicate with each other to exchange information about their current processor.

user case diagram network load balancing