The Adaptive Security Algorithm (ASA) uses security levels to describe whether a given firewall interface is inside (trusted) or outside (untrusted) relative to other interfaces. ASA security levels range from 0 (lowest) to 100 (highest), with 100 being the default for inside interfaces and 0 being the default for outside interfaces. Security levels 1 through 99 are typically used for interfaces connected to the demilitarized zone (DMZ).
In a typical configuration, inside interfaces are configured with higher security levels than outside ones. Packets entering the firewall through an interface with a higher security level can exit freely through one with a lower security level, while packets passing in the reverse direction are controlled by access lists or through a conduit.