When a node boots and starts Qnet along with a network driver, if that node is quiet (i.e., there are no applications on it that try to communicate with other nodes via Qnet), the /net directory is slowly populated by the rest of the Qnet nodes, which occasionally broadcast their node information.
The default time interval for this is 30 seconds, and is controlled by the auto_add=X command-line option to lsm-qnet.so. So, 30 seconds after booting, /net is probably as full as it's going to get.
When there's an entry in the /net directory, all it means is that Qnet now has a mapping from an ASCII text node name to an Ethernet MAC address. It speeds up the node resolution process ever so slightly, and is convenient for people to see what other nodes might be on the network.
Entries in /net aren't deleted until someone tries to use them, and they're found to be invalid.
For example, someone might have booted a node an hour ago, run it for a minute, then shut it down. It will still have an entry in the /net directories of the other Qnet nodes, if they never talk to it. If they did talk to it, and establish session connections, everything will eventually be torn down as the session connections time out.
To flush out invalid entries from /net, type:
ls -l /net &
To completely clean out /net, type:
rmdir /net/*