The term "packet filter" originated in the context of BSD operating systems.
Network layer firewalls generally fall into two sub-categories, stateful and stateless.
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Firewalls are often categorized as either network firewalls or host-based firewalls.
Application firewalls accomplish their function by hooking into socket calls to filter the connections between the application layer and the lower layers of the OSI model.
Application firewalls that hook into socket calls are also referred to as socket filters.
Firewall appliances may also offer other functionality to the internal network they protect, such as acting as a DHCP The host-based firewall may be a daemon or service as a part of the operating system or an agent application such as endpoint security or protection. Network layer firewalls, also called packet filters, operate at a relatively low level of the TCP/IP protocol stack, not allowing packets to pass through the firewall unless they match the established rule set.
The firewall administrator may define the rules; or default rules may apply.
Packets may be filtered by source and destination network addresses, protocol, source and destination port numbers.
The bulk of Internet communication in 20th and early 21st century used either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) in conjunction with well-known ports, enabling firewalls of that era to distinguish between, and thus control, specific types of traffic (such as web browsing, remote printing, email transmission, file transfer), unless the machines on each side of the packet filter used the same non-standard ports.