1. Network Address Translation (NAT):

• Many devices are behind NAT, where the router or firewall does not forward inbound traffic to the node.

2. Firewalls:

• Some nodes are configured with firewalls that block incoming connections for security or privacy reasons.

3. Tor or VPN Use:

• Nodes using the Tor network or VPNs often keep their IP addresses private, making them unreachable to the broader network.

4. Configuration Settings:

• Bitcoin Core and other implementations can be configured to disable incoming connections, limiting the node to outbound communication only.

5. Dynamic IPs or Private Subnets:

• Nodes with frequently changing IP addresses or those operating in private subnets may not be discoverable by others.

How Unreachable Nodes Contribute:

Unreachable nodes still download and verify blocks and transactions, contributing to the network’s decentralization and security by:

• Validating the blockchain independently.

• Relaying transactions and blocks to the reachable nodes they connect to.

However, they do not directly contribute to the propagation of new transactions or blocks beyond their connected peers.

Detecting Unreachable Nodes:

Detecting these nodes is difficult because they don’t publicly announce their presence. However, researchers estimate their numbers based on indirect observations, such as:

• Outbound traffic patterns.

• Passive monitoring techniques like the Passive Announcement Listening (PAL) method.