Packet-Filtering Firewalls
Packet-filtering firewalls examine the header information of data packets that come from a network. The restrictions most commonly implemented are based on a combination of:
- Internet Protocol (IP) source and destination address
- Direction (inbound or outbound)
- Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) source and destination port requests
Simple firewall models examine one aspect of the packet header: the destination and source address. They enforce address restrictions, rules designed to prohibit packets with certain addresses or partial addresses from passing through the device.
They accomplish this through access control lists (ACLs), which are created and modified by the firewall administrators. There are three subsets of packet-filtering firewalls:
- Static filtering
- Dynamic filtering
- Stateful inspection
Static filtering requires that the filtering rules governing how the firewall decides which packets are allowed and which are denied are developed and installed.
Dynamic filtering allows the firewall to react to an emergent event and update or create rules to deal with the event.
While static filtering firewalls allow entire sets of one type of packet to enter in response to authorized requests, the dynamic packet-filtering firewall allows only a packet with a source, destination, and port address to enter through the firewall.
Stateful inspection firewalls, or stateful firewalls, keep track of each network connection between internal and external systems using a state table, which tracks the state and context of each packet in the conversation by recording which station sent which packet and when.
Whereas simple packet-filtering firewalls only allow or deny certain packets based on their address, a stateful firewall can block incoming packets that are not responses to internal requests.
Primary Disadvantage: this type of firewall requires is the additional processing to manage and verify packets against the state table, which can leave the system vulnerable to a DoS or DDoS attack.

Whether you're preparing for Cybersecurity certification, working with government standards, or simply starting your career in compliance, these are the NIST Federal Information Processing Standards (FIPS), Special Publication (SP), and Interagency Report (IR) topics