Bitnodes estimates the relative size of the Bitcoin peer-to-peer network by finding all of its reachable nodes. The current methodology involves sending 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 (i.e. >= /Satoshi:0.8.x/), so nodes running an older protocol version will be skipped. The crawler implementation in Python is available from GitHub (ayeowch/bitnodes).
The crawler maintained by Bitnodes connects from these IP addresses: