To me, NFS is like black magic. Getting it to work consists of trial and error and feel lucky afterwards.

The big plan was to use a /usr/ports directory from another machine on the same network. Shouldn't be too hard, right? Just adding a line in the server's /etc/export:

But alas, when mounting on the client, the following error appears:

After some Googling it seems that the line in export requires an IP address. I don't know why, since other lines don't need one. But adding the IP of the client did the trick:

Not really flexible, I experimented with the -network and -netmask options, but that made the error appear again.