Subnetting Question

[tm1776]

Hi there. Here's a question for some of you network Gurus. I am a Uni student taking a degree in Computer Networking. I do not have much knowledge on subnets, and IP addresses, and them sort of topics Not as much as I should. I do struggle with it. But anyway. I ran into this question;

"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

 

[riser]

He's correct.

100.200.4 is the subnet, 200.4 is more precise. When using .0 in IPs, that generally means the entire range. Looking at the, the subnet would be 255.255.255.0 most likely.

100.200.4 is actually the subnet, .0 to .254 are the available IP addresses to give to the host, .255 is the broadcast IP.

100.200.4.0 means the entire range. If subnetted with X amounts of subnets, you would break that 100.200.4.x range up into host IP groups.

2 subnets of it would be like .1 to 122 and .123 to .254. That would create 2 subnets on 100.200.4.0 subnet. Subnet ID would be like 255.255.254.xxx I think.

 

[calyn]

The question "What does 100.200.4.0" refer to?" is actually incomplete. If used in a classful environment the asnwer is "An IP address". If it is used in a classless environment the only good answer is "That depends (on the subnet mask)".

 

[riser]

No. Anytime an IP address ends in .0 it means the entire range.

You are given 100.200.4.0 for your subnet range. Meaning, you get them all. Now if they said like 100.200.4.0 subnet 255.255.x.x or any variable in there, it would change the range.

 

[aalever]

"Anytime an IP address ends in .0 it means the entire range"

This isn't correct.

If the network is 10.200.4.0/8 (i.e. with mask 255.0.0.0), then 100.200.4.0 is a usable host in the range from 10.200.4.1 - 10.255.255.254.

If the network is 10.200.4.0/24 (i.e. with mask 255.255.255.0) then 100.200.4.0 is the subnet address.

So calyn is correct - it depends on the subnet mask.

Back in the day, when people thought that there would be enough IP addresses in a 32 bit scheme to go round for everyone, they developed the concept of class A, B, and C networks to give a network administrator an addressing scheme with a variable number of hosts allowed at different prices for the public range. Back then, you could say that an IP beginning with 10 (in the class A range) would have 2^24 -2 usable hosts (for net and broadcast addresses). Nowadays, since subnetting was introduced, that statement is no longer accurate because it entirely depends on how that network has been subnetted. That could have been the point your lecturer was trying to make.

HTH

(I realise that this was posted a few years ago but if you, like me, found this recently then I hope to clear up any confusion. plus, I'm a pedant )

 

[aalever]

Sorry, this:

"If the network is 10.200.4.0/8 (i.e. with mask 255.0.0.0), then 100.200.4.0 is a usable host in the range from 10.200.4.1 - 10.255.255.254. "

should read this:

"If the network is 10.200.4.0/8 (i.e. with mask 255.0.0.0), then 100.200.4.0 is a usable host in the range from 10.0.0.1 - 10.255.255.254. "