Exactly how many Class A, B and C networks exist?

Hello enda-ga,

Your question has some complexity to it.  For example, some subnets
are "private", meaning they are not routable.  You can use them inside
your own network, but not on the internet.  So, the 192.168.x.y series
of subnets could be counted once, but since anyone can use them, there
are actually thousands of copies in existence.  Also, subnetting
confuses the issue somewhat; a class B may be subnetted into blocks
the size of class Cs: how should those be counted?

The simplest answer is to refer to RFC 1166: Internet Numbers (
http://www.faqs.org/rfcs/rfc1166.html ).


The relevant sections are:

"The first type of address, or class A, has a 7-bit network number and
a 24-bit local address.  The highest-order bit is set to 0.  This
allows 128 class A networks."

"The second type of address, class B, has a 14-bit network number and
a 16-bit local address.  The two highest-order bits are set to 1-0. 
This allows 16,384 class B networks."

"The third type of address, class C, has a 21-bit network number and a
8-bit local address.  The three highest-order bits are set to 1-1-0. 
This allows 2,097,152 class C networks."

You may be interested to know that there exist class D and E networks
as well.  The RFC covers this as well, and gives an interesting look
at network assignment circa 1990.

Search strategy:
> check Unix System Administration Handbook
"Number of subnets"
"Class network number"
"Network class RFC"

Thanks for pointing me in the right direction. I've only just started
studying networks myself - its a new area for me. Thanks again.