please don't - this is a "complex topic", to put it mildly.
See, for instance, RFC 1775 on the definition of being "on" the Internet.
The closest thing to an authoritative source I know is the Network
Wizards/IDC domain survey - www.nw.com - counting the number of hosts
registered in the DNS.
Good luck - but don't ask the IETF community unless you're particularly
fond of speculations!
Allen Sears at CNRI asked if we might be able to answer the questions
below. Shall I forward this on to the IETF community to solicit
input? Any assistance is appreciated.