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## Forum Replies

1. Hi Mario,

The first two bits have to be 10 so that means the following range is class B:

10000000 - 10111111

Or in decimal:

128 - 191

Rene

2. Regarding the prefix that are covered in 128.0.0.0/3 & 128.0.0.0/5.
Why does it stop at subnet #6 for /3 and #19 for /5

256 – 224 = 32 subnets
256 – 248 = 8 subnets

So for /3 it looks like this:

Subnet #1: 0.0.0.0/3
Subnet #2: 32.0.0.0/3
Subnet #3: 64.0.0.0/3
Subnet #4: 96.0.0.0/3
Subnet #5: 128.0.0.0/3
Subnet #6: 160.0.0.0/3
And for /5:

Subnet #1: 0.0.0.0/5
Subnet #2: 8.0.0.0/5
Subnet #3: 16.0.0.0/5
Subnet #4: 24.0.0.0/5
Subnet #5: 32.0.0.0/5
Subnet #6: 40.0.0.0/5
Subnet #7: 48.0.0.0/5
Subnet #8: 56.0.0.0/5
Subnet #9: 64.0.0.0/5
Subnet #10: 72.0.0.0/5
Subnet #11: 80.0.0.0/5
Subnet #12: 88.0.0.0/5
Subnet #13: 96.0.0.0/5
Subnet #14: 104.0.0.0/5
Subnet #15: 112.0.0.0/5
Subnet #16: 120.0.0.0/5
Subnet #17: 128.0.0.0/5
Subnet #18: 136.0.0.0/5
Subnet #19: 144.0.0.0/5

3. Ok, Hi everyone just joined the site.

I want to make sure that my brain understands this. It’s like you slice and dice and hopefully everything comes out right. So say I have this already as my prefix list:

10.0.12.0/24 le 32

Now say I want to include another network. So I want the following two networks in one prefix-list.

10.0.12.0/24 and 10.0.13.0/24, I want these covered by one prefix list.

So I figure that this will fit within the scope of these two networks.

2 will be size of subnets:

0, 2, 4, 6, 8, 10
12 and 13 — this fits just right.
14, 16, 18 etc…

So I delete the old prefix and add this:
10.0.12.0/23 le 32

I’m not worried about the “le 32” as that basically means I’m accepting all addresses in the 10.0.12.x and 10.0.13.0 scope.
So do I understand this correctly?

Thank you!

4. Hi Michael,

Seems you got it right yes:

`10.0.12.0/24 le 32`

This will match all 1.0.12.X networks that have a subnet mask of /32 or larger (like /31, /30, /29, etc.).

With this one:

`10.0.12.0/23 le 32`

You have everything that falls within 10.0.12.0/23 range and with a subnet mask larger than /32 (/31, /30, /29, etc.).

Rene

5. I was so confused by this at first. I know its simple but its also a brain teaser for some reason.
first I pulled up my boson subnet tool its free tool by the way on the Boson.com (just need to create an account) website. At first I was thinking that first two bits mean the first two spots _ _ thinking it could be anything from 0 to 192 that it did not matter if it was a zero or a one. However after putting into the subnet calculator it helped me to see.

The next really helpful thing for me was when I went ahead and enabled the command with distribute-list prefix CLASSB in all of a sudden all my 10.x.x.x networks disappeared which shot my first theory to crap along with supporting the boson subnet calculator.

So seeing it in play in a lab really made sense. I think the confusion goes back to the rule on how the classes are setup. When we was learning sub-netting classes that you read and say oh ok but as time goes on you just get use to seeing the numbers themselves and that they are a certain class.

I am betting everyone that had a problem with this does not use the Class A, B, C rule anymore but instead over time have subconsciously just memorized the 1-127 is A, 128-191 is B, 192-223 is C and does not really think of the rule about class A the first bit always being 0, and class b the first two bits being set to 10, and class C having its first three bits set to 110… (hoping me explaining this in writing will actually help me remember it! lol)

So its like trying to do a math problem and finally that silly rule in math never used much is key to how the expression functions.