A layer-$4$ firewall (a device that can look at all protocol headers up to the transport layer) $\text{CANNOT}$
- block entire $\text{HTTP}$ traffic during $9:00\; \text{PM}$ and $5:00 \; \text{AM}$
- block all $\text{ICMP}$ traffic
- stop incoming traffic from specific $\text{IP}$ address but allow outgoing traffic to the same $\text{IP}$ address
- block $\text{TCP}$ traffic from a specific user on a multi-user system during $9:00\; \text{PM}$ to $5:00\; \text{AM}$
