Open problem
Full nodes behind NAT can't serve
open
Many full nodes run on home connections. The router in front of them permits outbound connections only, so nothing outside can dial in.
These nodes hold the data and verify the chain, but no reader can reach them. They read the network without serving it.
libp2p provides AutoNAT to detect reachability, DCUtR for hole punching, and relays as a fallback. Every node those tools make reachable is new permissionless supply. See connectivity.