Hi. I couldn't find a section on Networking. I have a subnetting Question.

I ran into this question in preperation for an exam;

"What does 100.200.4.0" refer to?" <<<-- pretty vague question I know, but the answer according to our lecturer is

"The entire Subnet ranging from 100.200.4.0 to 100.200.4.255"


So what confused me about this answer is the fact that 100. is a Class A address scheme. So since it is Network.Host.Host.Host, why is the last octet been assigned the subnetID bit? Is that not the case for Class C?


I would of thought that the second octet would be the subnet identifier. In our Uni we have set up our network and it is Class B. Our Third octet is our unique Subnet that we're on. That makes sense because it's something like "172.16.|128|.0"
and our mask is 255.255.248.0 and we are the 16th usable subnet out of a total of 32.

With the 128 being unique to us, and since the 172.16 is the network part and cannot be changed then It does make sense that our next available octet has been used for the subnet.


So my question is this. Is there a set way in what octet is your Subnet Bit according to the class? I just figured that in a Class A network, the answer for the question listed above would be

"The entire Subnet ranging from 100.200.4.0 to 100.255.4.0"

I hope I have made myself clear enough here. My apologies If I haven’t. I appreciate responses very much. Thanks

I understand what you're saying, and it's just that your logic is off. Well, that and the fact that 100.200.4.0 is just a literally randomly picked address.

Generally, you want to keep your class structure using powers of 2 (2, 4, 8, 16, 32, etc.). That's why I say 100.200 is just random.

A Class A network would be 10.0.0.0/24, the '/24' meaning that 24 bits are used to specify unique addresses. As you go from Class A to B to C to D, the '/x' goes down, so Class B is 10.10.0.0/16, C is 10.10.10.0/8 and Class D is, well, a unique IP.

100.200.4.0 - the '.0' on the end is what lets you know that this is where the pool of unique addresses will come from, so the previous number of octets will identify the Class. Since you have 3 used octets (100, 200, and 4), it's a Class C address. This should tell you that the address range is of Class C, which is 100.200.4.0 - 100.200.4.255.

If the address in question were 100.200.0.0, then you'd recognize that as a Class B.

Again, that address...totally random. You would never see that used anywhere but in a private network, and probably wouldn't even see it there.

I hope that helps. I'd done that for about 2 years straight at my last job and it's just completely fried my brain.

You're right. It is a Class A subnet but apparently it's been subnetted further into 254 host nets (or just one 254 host subnet and the rest being VLSM). That's what the 0 in the last octet means. The proper CIDR form would be 100.200.4.0/24. There is nothing that would stop you from taking a big subnet and breaking it up up into smaller ones as you see fit.

BTW.
This is not correct if you were dealing with a whole Class A network: "The entire Subnet ranging from 100.200.4.0 to 100.255.4.0". The right answer would have to be 100.0.0.0 through 100.255.255.255. There is no address range that goes from 100.200.4.0 to 100.255.4.0. Remember that the dotted decimal representation of subnet masks and IP addresses is a logical one. It all looks different in binary form.