According to the data from Bitnodes, in the current year, the number of reachable nodes on the Bitcoin blockchain fell 18.9% and the number of unreachable nodes fell 33%.
Bitnodes is developed to estimate the size of the Bitcoin network by finding all the reachable nodes in the network. Bitnodes sends getaddr messages recursively to find all the reachable nodes in the network, starting from a set of seed nodes. Bitnodes uses Bitcoin protocol version 70001 so the nodes which are running an older protocol version will be skipped. According to the most recent data by Bitnodes, the number of reachable nodes on Bitcoin Blockchain fell from 11,845 to 9,597 and in the same period, the number of unreachable nodes also fell from 98,000 to 65,500.
On the Bitcoin blockchain, the nodes which are able to send and receive connections from Bitcoin network are called reachable nodes while the nodes which can only make outgoing connections but cannot accept incoming nodes are known as unreachable nodes. Unreachable nodes cannot accept incoming connections because either they are behind a firewall or shielded by a Network Address Translation (NAT). NAT modifies the network address information in the IP header of data packets while they are in transit through a router.
A falling number of reachable nodes can lead to increased centralization of the network. If the number of reachable nodes falls at the same pace, over the time a fewer number of entities will start to control the remaining full nodes. If it reaches 51%, the risk of attacks and double spend will increase and the credibility of Bitcoin will destroy. Though this kind of condition is not likely to happen but because of the continued bear market crypto miners are leaving the space, driven by low or negative profit margins. Decreasing interest in cryptocurrency mining is directly affecting Bitcoin. Recently the difficulty level of Bitcoin mining is adjusted downward by 7% so that a stable hash rate can be maintained.
Withdrawal of several small miners will lead to a drop in reachable node numbers. If we talk theoretically the increasing number of unreachable nodes are good because it makes the network more secure by increasing the number of obstacles in the way of hackers. Reachable nodes play an important role in the Bitcoin protocol because they can function as a full node and can keep complete records of all blockchain transactions from genesis which maintains the integrity of the Bitcoin blockchain.