8
NAPs, POPs, ISPs e NSPs
http://nap.aads.net/main.html
http://www.ansp.br/ptt/
http://www.cg.org.br/grupo/operacao_ptt_v1.1.htm
http://www.bigeast.net/html/big_east.html
The first four Tier-1 NAPS are all in the United State
San Francisco, Chicago, Washington and New York -- and were built in conjunction with the National Science Foundation.
http://www.napoftheamericas.com
http://www.netconfigs.com/general/ranks.htm
Historically, the different networks that together make up the Internet were grouped into tiers.  A Tier1 was one of the original backbone networks, a Tier2 was a network connected to Tier1 and so on.
The growth of the Internet and the resultant increased mesh-ed-ness of all the networks involved means that traditional tier classifications are less meaningful.