Packet Filtering
Packet filters are an effective element in any perimeter security setup. In addition, they have the advantage of not taking up bandwidth, or the capacity of network cables to convey information, the way proxy servers do.
A packet, which is sometimes called a datagram, contains two types of information: the header and the data. Packet filters use packet headers to decide whether to block the packet or allow it to pass through a firewall. Note that your job as a system administrator would be to configure the firewall to deny all packets that arrive from outside but contain a source IP address that seems to be coming from within the network.
Stateless Packet-Filtering Firewalls
Stateless inspection, also called stateless packet filtering, is firewall packet inspection that ignores the state of the connection between the internal computer and the external computer. A firewall that conducts stateless packet filtering simply blocks or allows a packet based on the information in the header.
Stateful Packet-Filtering Firewalls
Stateful inspection, also called stateful packet filtering, is an examination of the data contained in a packet as well as the state of the connection between internal and external computers. This information, known as the state table, is kept in a memory location called the cache. Stateful inspection is superior to stateless inspection because it uses the connection state to make decisions on whether to allow the traffic
Packet-Filtering Rules
- Some of the most general packet-filtering rules include:
- Any outbound packet must have a source address that is on your internal network.
- Any outbound packet must not have a destination address that is on your internal network.
- Any inbound packet must not have a source address that is on your internal network.
- Any inbound packet must have a destination address that is on your internal network.
- Any packet that enters or leaves your network must have a source or destination address that falls within the range of addresses in your network.
Filter rules can affect the transmission of packets. These rules include the use of the following:
- Internet Control Message Protocol (ICMP)
- User Datagram Protocol (UDP)
- TCP filtering
- IP filtering

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