Besides filtering of IP applications, other technologies can easily be classified as firewalls, if they perform any access control between networks. Examples of such technologies include:
■ Filtering of Layer 2 (L2) frames, using a L2 device such as a dedicated switch or bridged router interfaces
■ Setting of static ARP entries or switch CAM entries, which effectively only enables communication between selected hosts
■ Filtering of voice/data calls on a PBX
Filtering of incoming ISDN data calls based on the Caller ID
■ Filtering of X.25 sessions based on caller or called party addresses
■ Translation of IPX/SPX protocols into TCP/IP, using a gateway that also restricts access
All these options can be used as standalone access control mechanisms or to complement existing methods to provide defense-in-depth.
A firewall can and usually is built as a hybrid of technologies:
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