Which of the following could be responsible?

An administrator is testing a new web server from outside of the corporate firewall. The administrator performs a test from a single PC and the web server responds accordingly. The administrator then provisions several virtual machines on a network behind NAT and uses them to perform the same operation on the web server at the same time, but thereafter soon discovers that none of the machines can reach the web server. Which of the following could be responsible?

A.
IPS

B.
Blacklisting

C.
IDS

D.
Whitelisting

E.
Firewall