BBN’s TCP implementation contained something akin to the hosts file, called hostmap there: https://www.tuhs.org/cgi-bin/utree.pl?file=BBN-Vax-TCP/doc I have not looked at the code for a while, but if I remember correctly the BBN kernel code also read in this file (pre-processed into a binary form) to build its internal routing table. I do not recall having seen an equivalent file with UoI's NCP Unix in any of the surviving docs or sources - but that does not exclude a library having existed to do lookups in a local copy the SRI-NIC host file. In fact there is some evidence for that in the 2.9 BSD source. The only surviving copy of the 4.1a (network) source code that I know is in the back-port of this code to 2.8/2.9 BSD. This code includes #ifdef’ed code for accessing the SRI-NIC online host table via NCP: https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/local This source also contains tools to convert the SRI-NIC data into - inter alia - a hosts file: https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/man/man8/htable.8 https://www.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/net/man/man8/gettable… It would seem that the modern host.txt on Unix evolved late ’81 (BBN code) to early ’82 (4.1a BSD). Possibly NCP Unix has prior work. Paul