Subnetting

[leo76]

I want to create a group of subnets using the following IP: 192.168.0.0. As far as I’m concerned, since the default subnet mask for this IP would be /24 because it is class C, I’m just allowed to touch the 4th octet to create my subnets. So, if I do the following, these subnets would be incorrect?

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

[Robert5205]

Yes, if the problem you were given makes the assumptions you’ve stated, those would be correct.

In the real world (not homework world), the class system has been replaced by “classless.” You could use any mask you wanted and no “classful” mask is assumed.

Now, having said that, entering an IP into Windows will still auto-fill the classful mask.

[leo76]

So, what you’re trying to say is that from a classful standpoint this would incorrect but from a classless standpoint it would be correct?

[spiceuser-1nzm7]

There is no default subnet mask anymore and no such thing as Class C, unless you can find a 25-year-old router from somewhere and get that working.

192.168.0.0/16 includes all ip addresses from 192.168.0.1 to 192.168.255.254, so there’s no such thing as a network called 192.168.1.0/16.

You can have
192.168.0.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24
or
192.168.0.0/23 192.168.2.0/23 192.168.4.0/23 192.168.6.0/23
or
192.168.0.0/22 192.168.4.0/22 192.168.8.0/22 192.168.16.0/22
etc…

[bbigford]

Yep your math is correct.

[jakelee4 (Jakedasnake)]

I think you’ve misread the previous answer, there still is a subnet mask but the classful defaults aren’t necessarily in use.

And FYI the classful defaults are (excluding D and E).

Class SM CIDR

A 255.0.0.0 /8

B 255.255.0.0 /16

C 255.255.255.0 /24

A classless subnet mask example is 255.255.255.192 or /26 in CIDR.

edited for typo

[leo76]

Oh no, I got what he meant. Just wanted to use another example with a different classless approach. But thanks for your feedback.

[Robert5205]

@jakelee4

I think you’ve misread the previous answer, there still is a subnet mask but the classful defaults aren’t necessarily in use.

And FYI the classful defaults are (excluding D and E).

Class SM CIDR

A 255.0.0.0 /8

B 255.255.0.0 /16

C 255.255.255.0 /24

A classless subnet mask example is 255.255.255.192 or /26 in CIDR.

edited for typo

If we’re going to get technical, a /26 is not a CIDR subnet mask - it’s just a subnet mask.

Even in the classful days, when you subnetted your IP domain, you still could use /26 as the mask on one of your subnets.

Classful routing is about so much more than just the mask. But, as mentioned, is so irrelevant now to actual routing that I cry inside a little every time I see a homework question based on it.

Classful routing is about assumptions. In the same way that your local telco/PBX assumes that a 7-digit number uses the local area code, for example. (As the concept of routing originated with the telco, this is particularly appropriate.) When the assumptions were removed from router programming, they became irrelevant.

[carlsposito (Hoodatdude)]

Short answer…Yes, that would be correct.

[spiceuser-1nzm7]

@rhummel

@jakelee4

I think you’ve misread the previous answer, there still is a subnet mask but the classful defaults aren’t necessarily in use.

And FYI the classful defaults are (excluding D and E).

When the assumptions were removed from router programming, they became irrelevant.

As far as I am aware, they’re irrelevant to the point of not even existing.
Except in out of date textbooks, of course.