Bitnodes is currently being developed to estimate the size of the Bitcoin network by finding all the reachable nodes in the network. 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
BITNODES TAKES UP ONE CONNECTION SLOT FROM EACH REACHABLE NODE IN THE NETWORK. IF YOU INTEND TO USE BITNODES TO CRAWL THE NETWORK, PLEASE CONSIDER RUNNING ONLY 1 INSTANCE OF THE CRAWLER TO AVOID TAKING UP THE VALUABLE CONNECTION SLOTS FROM EACH NODE.