OpenVMS doesn't use a hosts file that can be directly edited. You have to use either the
UCX (for pre v5 tcpip services) commands e.g.
tcpip> SET HOST test.example.com /address=10.0.0.100
tcpip> SET HOST test.example.com /alias=somealian
Would create a host entry for a host called
test.example.com with address
10.0.0.100 and an alias of
somealias. As with most things OpenVMS the command's internal help is excellent.
It looks like the file that contains the actual entries is pointed to by the logical name TCPIP$HOST.
The Documentation can be found online here.