This now-obsolete read-create object held the ifIndex of the loopback interface whose IP address an unnumbered interface built from this profile would borrow as its source address, with zero meaning none was specified. It let admins give unnumbered interfaces, which have no IP address of their own, a stable source address; it was later replaced first by juniIpProfileLoopback and then by juniIpProfileInheritNumString. For example, setting juniIpProfileLoopbackIfIndex to the ifIndex of loopback0 meant an unnumbered interface using this profile would transmit packets sourced from loopback0's IP address.