Identifies the IP interface on the firewall to which this session table entry belongs, and serves as part of the row's index alongside direction and session number. It lets an admin correlate a tracked firewall session with the specific network interface, such as internal versus external, that its traffic passed through, useful when diagnosing which interface a suspicious session used. For example, an index of 2 might correspond to the firewall's external WAN interface, distinguishing sessions arriving from the Internet from those tracked on the internal LAN interface.