Why do intranets use the IP address ranges 192.168.*.* and 172.16.*.* ?

Answer 1

The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

These will not be used on a global IP network

RFC1918 faq
http://www.faqs.org/rfcs/rfc1918.html



The rest may give you two much info.

Of the three "classes", class C would be the most frequently used. People often use this to refer to any network with a 24-bit subnet mask and 8-bit host address, although properly, it includes only those network addresses that begin with 110.



http://www.idevelopment.info/data/Networking/Networking_Basics/ROUTERS_IP_Subnetting.shtml

http://www.swcp.com/~jgentry/topo/unit3.htm

Answer 2

Those networks are reserved for internal use by IETF document RFC1918, a published standard. This helps isolate your internal network from the regular internet and keeps you from picking numbers that would collide with the real-world counterparts. The "real" internet cannot and will not route IPs in those ranges. Note that 10.*.*.* is also in this set.

Answer 3

Those ranges conform to the standards.

There are different classes of IP ranges and each range/class are meant for certain networks.