A darknet, for Freenet purposes, is a network where node connections are created manually by the users of the nodes via invitations. These connections should be made on the basis of mutual trust, friendship, or acquaintance. If they are, then the network should have a
small world topology and routing will work, especially since darknet peers are more or less permanent. If the connections are made completely at random, then the network topology will be wrong, and routing will not work.
Security
A darknet is not
harvestable because your node reference will never be passed on by the network to any other node. Therefore, an attacker running a darknet node can only see those nodes which he has managed to get invitations to connect to.
Consequently, you are vulnerable primarily to the nodes you are connected to. Thus, it is a good idea to only connect to people you trust, if this is possible.
See Also
OpenNet - The alternative to a darknet
FreenetConnections - How to create darknet connections in Freenet
Freenet 0.7 Security