An administrator determines that an attack is taking place on the email server from a group of users on the same ISP. Which of the following is the BEST way to mitigate an attack on the network?

Packet filtering is a firewall technique that inspects packet headers (source/destination IP, port, protocol) and allows or denies traffic based on defined ACL rules. Since the attackers share the same ISP, their traffic originates from a common IP range, making IP-based packet filtering an effective way to block all attack traffic at the network perimeter without affecting other users.

Spam filtering inspects email message content and headers to block unwanted messages, but it does not block malicious network-level traffic or active attacks at the packet level.

MAC filtering works only at Layer 2 on the local network segment; MAC addresses are not preserved across routed Internet connections, making it ineffective against remote attackers.

A CSU (Channel Service Unit) is a hardware device for WAN line conditioning and has no capability to inspect or filter traffic based on attack patterns.