Subnet Speed Practice #2: Answers

Hi Albert,
Thanks for the post! I disagree, but that’s what these exercises are great for – give it a try, struggle a bit, and learn.
Thinking it decimal, whatever the subnet ID is, add 1 to the 4th octet to get the numerically lowest address.
Conceptually, I think you’re thinking about the /23 mask as creating 1 host bit in the 3rd octet, but ignoring the 8 host bits in the 4th octet.
The usable addresses in the subnet are 10.1.0.1, 10.1.0.2, .3, .4, etc to 10.1.0.255, then 10.1.1.0, 10.1.1.1, 10.1.1.2, .3, etc to 10.1.1.254 (last usable), the 10.1.1.255 as the subnet broadcast address.

Hope this helps.
Wendell

Hi Dayne,
The terms “classless addressing” and “classful addressing” confuse a lot of folks. It’s not that uncommon to find documents that differ in what they claim the terms to mean. I’ve been attempting to be consistent and clear about these terms in my books for 25+ years now, so if you have those, I encourage you to look at Chapter 13’s “Classless and Classful Addressing” section.
That said… the terms refer to a way to think. They do not change or impact how any devices behave. They are different views for us humans when we think about different aspects of IP addressing and subnetting.
Any time you think about Subnetting a class A/B/C network, you must think with classful addressing logic. That logic gives you a three-part view of the address structure: Network, Subnet, and Host.
On the other hand, when you are thinking about the structure as two parts, you are using classless logic. One part is the host part. The other is the prefix part, which is the part that has the same value for all addresses in a subnet. For example, when thinking about one specific subnet, you can think with classless addressing logic, as the division between network and subnet does not matter. When looking at the IP routing table on a router, you can think with classless addressing, thinking about which packets match each prefix (aka subnet.)

So, to your comments. Your paragraph that begins “This is where I’m confused…” correctly describes thoughts that use classful addressing logic. It describes a network. Unsubnetted, the structure has 8 network and 24 host bits. You then borrowed 15 host bits to create subnet bits. So you’re thinking about network, subnet, and host parts. That’s classful addressing.

Your paragraph beginning “As I understand it…” – I agree with the first sentence and it describes classless addressing. But I see the second sentence as either unrelated, or a thought that goes with classful addressing. That is, if you’re thinking about “borrowing bits”, then that is jargon associated with the subnet design process, starting with a classful network, which unsubnetting has a network and host part, and borrowing some of the host bits to create a subnet part. Three parts, so classful addressing logic.

Finally… I wouldn’t worry about the differences in meanings between the two terms. Honestly, understanding subnetting concepts, how to do the math, and so on is 100,000X more important than putting a label on whether you were using classless or classful addressing concepts. But since you asked, that dividing line is simply this: If you need to distinguish between the network and subnet parts, it’s classful addressing, and if you can ignore that distinction (and treat network+subnet as a prefix), then it’s classless addressing.

Hope your final month of study goes well. Keep at it!
Wendell