In its most basic form, a packet filtering firewall operates at which layer?

1. A basic packet filter looks only at addresses and ports. 2. That information lives low in the stack. 3. In their most basic form, firewalls with packet filters operate at the network layer. 4. So the answer is the network layer. _Source: NIST SP 800-41 Rev. 1, 'Guidelines on Firewalls and Firewall Policy'_