This read-only value indicates which entry in the standard MIB-II interface table this serial port corresponds to, with a value of zero meaning this serial port has no associated MIB-II interface at all, which is normal unless the port supports a protocol like SLIP or PPP. An admin checks this to know whether a given serial port can be monitored through standard interface counters or only through this MIB's serial-specific objects. For example, a serial port configured for PPP dial-up would show a non-zero ifIndex here, letting an admin monitor its traffic through the standard ifTable as well.