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A '''[[network layer]] [[firewall]]''' works as a [[packet filter]] by deciding what [[packet]]s will pass the firewall according to rules defined by the administrator. Filtering rules can act on the basis of source and destination address and on [[Port (computing)|port]]s, in addition to whatever higher-level [[network protocol]]s the packet contains. Network layer firewalls tend to operate very fast, and transparently to users.
Network layer firewalls generally fall into two sub-categories, [[stateful firewall|stateful]] and [[stateless firewall|non-stateful]]. Stateful firewalls hold some information on the state of connections (for example: established or not, initiation, handshaking, data or breaking down the connection) as part of their rules (e.g. only hosts inside the firewall can establish connections on a certain port).
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