For instance, I am dealing with NSFNET and DARPANET each having respective node degrees of 2.476 and 3.125. Are these considered low, average or high node degrees? Thank you.

It depends on the network. The numbers you give are, I suppose, the average node degrees for each, so DARPANET is somewhat “more connected” than NSFNET. Both would be quite robust with respect to broken links, since messages can probably be redirected around the break.

It would be possible to build a network with each node connected to all the others, giving a very high number, or to unfold that network to a single row of nodes, giving a result just less than 2. (Each node connects to its two neighbours, except for those at the ends.)

It would also be possible to get the same number with different topologies. The single row of nodes I mentioned above has about the same average degree as a central server with all the other nodes connecting there directly.

Thanks for the info.