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.
See Provisioning Bitcoin Network Crawler for steps on setting up a machine to run Bitnodes. The Redis Data contains the list of keys and their associated values that are written by the scripts in this project. If you wish to access the data, e.g. network snapshots, collected using this project, see API.