Subnetting

[PietroPoliseno27977]

Hi everyone, I noticed that there is a lot of confusion on the net about the concept of addressing; among other things, even in the official cisco books there is no answer to the following question: Considering that with classless the concept of classes should not be considered from the point of view of subnetting, can I use the address 192.168.6.0/16 for example? And if at that point it fails to consider 172.16.0.0 for larger networks I can also use 192.168.0.0 where 0.0 just following what classless says I can subnetting. Also I see on the internet that there are written private ip addresses 192.168.2.0 or 5.0 etc. etc. I have vaguely read but it is not clear the concept of the supernet that in the official Cisco books there is no argument.

[Martin L]

Yes, you can use /16 with 192.168.0.0;  172.16.0.0 /12, 10 .0.0.0/8

You cannot use anything less then mentioned above.  like  cannot use 172.16.0.0 /11 or /10; Or  192.168.0.0 /15

Most CCNA books have a copy of table from Rfc1918, Private Ranges with either network range or number or prefix length;  It is not mentioned that you could subent/suernet those ranges because section of Private Address space is in talk about Classes and at the beginning of addressing; nit subnetting or CIDR.  Besides, Rfc1918  Private Ranges was born before CIDR, classless network was used  (I think)

[Joseph W. Doherty]

Perhaps the distinction between allocation reservations for specific address blocks and classful vs. classless addressing isn't quite clear.

For the latter, classful vs. classless, what you need to understand, classful addresses were a way to pass along a network's prefix length without actually sending that as explicit data.  I.e. the IP address itself implied the prefix length.

Further, how this was done required only looking at the beginning bits of the IP addresses (early CPU sometimes, to see the status of a specific bit, had to "rotate or shift" the field, testing each bit, one at a time).  If the first bit of the IP address was a zero, it was a class A address and prefix length was /8.

If the first bit of the IP address was a one, but the second bit was a zero, it was a class B address and the prefix length was /16.

If the first and second bits of the IP address were ones, but the third bit was a zero, then the address was a class C and prefix length was /24.

If the first, second and thirds bits were all ones, then the IP address was a class D (multicast) or E address (determine by the fourth bit).

Subnetting, and less common supernetting, deal in subdividing (subnetting) a classful address into smaller network prefixes, or joining multiple (supernetting) classful addresses into a larger network.

After some time, it became clear it would be better to pass along a network's length, along with the network prefix, and classful became classless.  Technically, there's no longer subnetting or supernetting, but the term subnetting is still often use for any network prefix and/or the process of subdividing one address block.

Although with classless, we no longer subnet (or supernet), for backward compatibility, we still maintain certain address block usage reservations.  So, to your question, can you use 192.168.6.0/16 (incidentally, perhaps better shown as 192.168.0.0/16).

Well, it depends on how you wish to use it because 192.168.0.0/16 is defined as a private address block.  For such purposes, i.e. private, yes you can use it as a single /16, two /17s, one /17 and two /18s, etc.  Basically you can slice and dice it any way you desire.  I.e. you're not bound to use the 256 /24s, and only /24s, that are within the /16.

However, if you asked could you use 192.168.0.0/15, well now your jumping address allocation reservation boundaries.  From a classless perspective, you can, but from an address reservations perspective, probably not (as you're now mixing private and pubic address blocks).