The following are the primary security levels created and used on the PIX firewall:
Security level 100 : The highest possible level, it is used by the inside interface by default. Using the trusted-untrusted terminology, this level is considered the most trusted.
Security level 0 : The lowest possible level, it's used by the outside interface by default, making it the most untrusted interface. Traffic can pass from this interface to other interfaces only if manually configured to do so.
Security levels 1–99 : Can be assigned to any other interface on the PIX. On a three-pronged PIX firewall, the inside is typically 100, the outside is 0, and the third interface could be 50. Traffic from interfaces between 1 and 99 can pass through to the outside (0), but it is prevented from passing to the inside (100). This is because the interface has a lower security level setting than the inside.
