We pick d.root-servers.net, one of the 13 root servers (1), and ask it about mx16-g26.free.fr and learn that fr is delegated to five name servers (2), so we ask f.ext.nic.fr where we learn that free.fr is delegated to two name servers, including freens2-g20.free.fr (3), so we ask that one and get the following answer:
Free.fr is a domain controlled by two domain name servers at free.fr themselves. Both are on the same IP network. The primary name server is freens1-g20.free.fr. Incoming mail for free.fr is handled by two mail servers at free.fr having a total of four IP numbers. All four of them are on the same IP network. free.fr has one IP number (126.96.36.199).