ifindex

An ifindex, short for interface index, is a numeric identifier assigned to a network interface on a device. It serves as a stable reference used by the operating system, management tools, and network protocols to address an interface independently of its name.

In Unix-like systems, the kernel assigns a per-device interface index when an interface becomes active. The index is used by kernel data structures and by user-space tools such as netlink-enabled programs and the iproute2 suite. Linux exposes the current value at /sys/class/net/<iface>/ifindex, and tools like ip, ethtool, or dmesg can display it as part of their interface listings.

In network management contexts, ifIndex is defined by the IF-MIB as a unique identifier for interfaces on a device. It is used as the index into the ifTable, allowing management systems to reference interfaces consistently across protocols that may differ from the interface's human-readable name.

Stability varies: ifindex values reflect the current set of interfaces and may change if interfaces are added, removed, or renumbered. They are not guaranteed to be persistent across reboots or reconfigurations. On Linux systems, the loopback interface usually has the smallest index (often 1), with other interfaces assigned in ascending order as they are created.