Coincidentally, I'm reading Where Wizards Stay Up Late about the development of ARPANet and it says: > In designing the IMPs, BBN made it possible to loop the

Coincidentally, I'm reading Where Wizards Stay Up Late about the development of ARPANet and it says: > In designing the IMPs, BBN made it possible to loop the host and modem interfaces of the machine, so they could conduct “loopback” tests. The loopback test, which could be performed remotely, connected an IMP’s output to its input, effectively isolating the IMP from the rest of the network. This generated test traffic through the interface and allowed BBN to check the returning traffic against the outgoing traffic generated by the IMP. So it seems like it was in there from the very beginning. No mention of the 127/8 address specifically though.