I'm curious, how many lightweight nodes are there on the network? Is this easy to tell?
It isn't easy to tell for a number of reasons.
- The network protocols don't explicitly distinguish between full-nodes, SPV wallets or other types of software that communicate with full nodes.
- Many lightweight wallets only connect to predetermined non-public servers, as Murch pointed out.
- Just finding approximately all the current nodes in the network is itself difficult
- Some nodes are only intermittently present. Consider a node that is only connected for 10 minutes a week.
- The network is a gossip network. It isn't intended that anyone needs a list of all nodes. Therefore finding all nodes isn't explicitly catered for.
- Many nodes are likely to be relatively passive and may not accept inbound connections.
- Privacy is an important issue. This probably affects the amount of information leaked by software - even if that information seems innocuous to many.
- Nodes that use security measures such as Tor may be hard to count accurately - they might each appear as multiple nodes.