🚀 go-pugleaf

In article <2l8fcr$5kc@medusa.aom.bt.co.uk> stevef@aom.bt.co.uk (Steve Fosdick) writes:
>Ken Mintz (mintz@cup.hp.com) wrote:
>> Mark Landin (landin@cherokee.nsuok.edu) wrote:
>> > Can you subnet a Class C address, say by using a mask like 255.255.255.128?

>>   Not 128 specifically, but certainly other values, such as 192, 224 etc.
>>   The problem with 128 per se is that that creates a one-bit subnet id.
>>   Since all-zero and all-one values have special meaning, subnet id and host
>>   id fields must comprise at least __two__ bits each.

This is correct.

>We used 255.255.255.128 successfully, but we are already subnetting
>because our adress is actually class B.

....and since you have Class B address space instead of Class C, you're
using a nine-bit subnet mask, which is fine. The first two bytes are
your *network* mask in a Class B, while the remaining two are the real
*subnet* mask. In a Class C network, three bytes are the network mask,
and all subnetting must be done in the final byte. (Yes, a Class A network
has one byte of network mask, and can use most of the remaining three for
subnetting.)

Subnetting is done using bits within your address space--the bits over the
assigned network aren't included. This is somewhat confusing, unfortunantly;
masks are interpreted differently depending upon what type of network they're
being used in. They break down like this:

		   (decimal)			     (binary)
Mask:		255.255.255.240		11111111.11111111.11111111.11110000

Class A:	NNN SSS SSS S/H		NNNNNNNN SSSSSSSS SSSSSSSS SSSSHHHH

Class B:	NNN NNN SSS S/H		NNNNNNNN NNNNNNNN SSSSSSSS SSSSHHHH

Class C:	NNN NNN NNN S/H		NNNNNNNN NNNNNNNN NNNNNNNN SSSSHHHH

Where "N" indicates network mask, "S" indicates subnet mask, and "H" is host
address bits. So, the mask 255.255.255.240 would provide a 20-bit subnet mask
in a Class A network, a 12-bit in a Class B, and only a 4-bit on a Class C.





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Gary Heston      SCI Systems, Inc.       gary@sci.com       site admin
   The Chairman of the Board and the CFO speak for SCI. I'm neither.
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