packet filter
termnetworkingsecurity
The simplest filtering: decide on each packet independently, by addresses, protocol and ports, with no memory of what came before.
Statelessness is both the appeal and the limit. It is fast, cheap and predictable, and it can be implemented in hardware at line rate - which is why access control lists live on routers and switches rather than only on security appliances. What it cannot do is tell a reply from an unsolicited packet, because that requires remembering the request. Permitting the return traffic therefore means permitting a whole class of packets that merely look like replies, which is the gap stateful inspection was invented to close.
Also known as: acl, access control list