Network layer firewall

Firewall operating on the network layer of the Protocol stack, working as a packet filter which decide what packets to pass the firewall according to rules defined by the administrator.

Filtering can be applied based on source and destination address, ports, in addition to what higher level protocols the packet contains. Stateful firewalls can also decide wheter to pass the packet on or not depending on the state of the connection (Initiation, handshaking, data or breaking down the connection).

Network layer firewalls tend to be very fast and transparent to users, as they are quite similiar to a Router. Actually most modern routers have some packet filtering capabilities.

Network layer Firewalls generally fall into two sub-categories, stateful and non-stateful. Stateful firewalls also hold some information on the state of connections (i.e. established or not) as part of their rules (e.g. only hosts inside the firewall can establish connections on a certain port).