A server or computer can protect itself by blocking malicious computers using a firewall. A firewall keeps a list of "bad" source addresses and drops all packets that come from an address on the list. In this example, Google has learned from the previous attack that Alice is a malicious user and has set up a firewall to block her. Observe how the packets from Alice's computer to Google are dropped.
One way for an adversary to bypass a firewall is to launch a Distributed Denial of Service (DDos) attack. To do this, the adversary sends packets from multiple machines under her control.
To pass this level, launch a DDoS attack against Google from Alice's additional machines.