Stateful Vs Stateless Firewall

Stateless Firewall watch network traffic and block it based on source and destination addresses or static values.

When a stateful Firewall allows an outgoing connection it knows that it has to allow the return connection for the same session. There is no need to create an access rule. Only one outbound connection rule is enough.

Whereas stateless firewall needs two different rule one for outbound connection and one for return connection for the same session. They are basically ACLs and contain rules about which traffic to allow/block depending on Source IP, Destination IP, Port numbers, Protocols etc. But it can be vulnerable and different attack techniques might bypass them.

All modern firewalls come with stateful inspection by default. Its also called Stateful Packet Inspection.