Quick question about Subnetting

[RedString]

Say I have reg. Class C IP (192.168.15.0) with 255.255.255.0 and I borrow 2 bits turning it into 255.255.255.192 (192.168.15.0/26)

I'll end up with 4 subnets with 62 possible hosts for ea.

So my question is, can I not use the 1st and last of the subnets since they contain the network & broadcast numbers?

ie: Subnet that includes the .0 and .255

Is formula 2^N-2=useable subnets?
N being number of bits borrowed

Kind of like hosts is 2^N-2=useable hosts
N being number of bits left for host

Just wondering cause I thought that was how it was, yet this Cisco course text mentions in one spot that if we need 7 subnets, we borrow 3 bits. And in another spot it says to use the first available block and lists it as .0-.26 (First block in the subnet).

Ty

 

[her209]

You can't use it if the no ip subnet zero command is in effect.

 

[her209]

Just wondering cause I thought that was how it was, yet this Cisco course text mentions in one spot that if we need 7 subnets, we borrow 3 bits. And in another spot it says to use the first available block and lists it as .0-.26 (First block in the subnet).

On this, if you borrow 3 bits for subnet, then that leaves 5 bits for host. That means subnets go up every 32. That is:
192.168.15.0 /27
192.168.15.32 /27
192.168.15.64 /27
192.168.15.96 /27
192.168.15.128 /27
192.168.15.160 /27
192.168.15.192 /27
192.168.15.224 /27

Assuming you are allowed to use the zero subnet, the assignable IP addresses in the first subnet is 192.168.15.1 - 192.168.15.30.

HTH

 

[theevilsharpie]

So my question is, can I not use the 1st and last of the subnets since they contain the network & broadcast numbers?

The zero subnet and all-ones subnet have always been usable.

 

[RedString]

Hm, I thought you weren't allowed to use the subnet that had (in your #'s) 192.168.15.0, because this included the network number. Same with ip that includes 192.168.15.255 because that is the broadcast number. If you tried to broadcast to your subnet only, it would end up broadcasting to the entire network, even the ones outside your subnet.

I'm pretty sure I remember my teacher drawing a box, cutting it into portions to show subnets, writing the network & broadcast in topleft/botright of each subnet box - but cutting out the first and last subnet due to the fact of not being able to use it. (for reasons like broadcast mentioned above.)

I'm talking about classful addressing by the way. I've yet to start on classless so I'm not sure if that would change things or if that isn't obvious already. I'm not sure why there's much focus on it really, classful I mean, I've read that it stopped being used much in the 90's. From what I do know, it seems like classless would typically be the way to go - so why bother with classful.

 

[theevilsharpie]

Hm, I thought you weren't allowed to use the subnet that had (in your #'s) 192.168.15.0, because this included the network number. Same with ip that includes 192.168.15.255 because that is the broadcast number. If you tried to broadcast to your subnet only, it would end up broadcasting to the entire network, even the ones outside your subnet.

I'm pretty sure I remember my teacher drawing a box, cutting it into portions to show subnets, writing the network & broadcast in topleft/botright of each subnet box - but cutting out the first and last subnet due to the fact of not being able to use it. (for reasons like broadcast mentioned above.)

The use of the zero subnet used to be discouraged because of the confusion that resulted from having a classful network and a subnet of that network with the same ID. The use of the all-ones subnet was discouraged because a misconfigured subnet mask in your routing table has the potential to cause a routing loop.

However, while their use was once discouraged in the distant past, it was never a hard limitation. If your equipment supported it, you could always make use of the zero subnet and all-ones subnet and still be fully IP compatible.

I'm talking about classful addressing by the way. I've yet to start on classless so I'm not sure if that would change things or if that isn't obvious already. I'm not sure why there's much focus on it really, classful I mean, I've read that it stopped being used much in the 90's. From what I do know, it seems like classless would typically be the way to go - so why bother with classful.

Classful addressing hasn't been relevant since the mid-90s. I have no idea why it's still taught in anything other than a historical context.

 

[MtnMan]

Classful addressing hasn't been relevant since the mid-90s. I have no idea why it's still taught in anything other than a historical context.

I think learning the basics of classful subnetting provides the foundation of how subnets relate to each other before VLSMs are introduced.

 

[RedString]

Well, all my questions were answered today during class since we're now (finally) talking about classless & ...crap I can't remember the name of it, VLAN Vsomething, it's subnetting subnets pretty much. So yeah, it was just some classful crap like you said, older devices had confusion problems when .0 subnet was used, but now it's fine. Though he is still recommending that we do not use .255, rather we would usually put out subnetted subnets (still dont remember correct term) at the end of our network, make them /30 so there is only 2 hosts in each subnet - that way .255 subnet is only losing 2 hosts by not being used and the other /30's can be used to address our router ports. So that fixes the problem of wasting a bunch of hosts.

well, just read MtnMan - VLSM's is the term I couldn't remember. & Yeah, Classful is pretty lame since no one uses it, there is a lot of complications n etc.. but I feel like it does give us a better foundation for moving into classless & VLSM. The only complaint I have is that my teacher didn't stress enough that these problems with classful isn't that important since no one uses it anymore. Hell I didn't even know no one used it anymore until I was reading up at home and noticed something mentioning it wasn't used much after 90s. & During class I would ask about why they did something like they did, and he would just say oh that is fixed with classless and we'll get into that later. And I would just think, why the heck are we talking about classful then if all these problems are already solved and it is outdated.

Now that I'm getting into classless, it makes a little more sense. Broader understanding and what not.