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Network Address Range - CIDR Notation - Explaination Please?

Hello All;

  We are fixing to correct a lot of bad Entries from our SMTP BLOCK LIST in our Mail Server.
The following information we are needing to find out.
===========================================================
IP Address
172.16.0.0 - 172.31.255.255  /12

If I was to enter the:   172.16.0.0        into our IP Field. And then Enter   172.31.255.255   into our MASK field.
Would that mean that it will look at and BLOCK out everything from    172.16.0.0 - 172.31.255.255

I am understanding that correctly?

10.0.0.0 - 10.255.255.255 /8   --> I add this in, and it BLOCKS everything in that range.
172.16.0.0 - 172.31.255.255 /12  --> This I am not sure about?
192.168.0.0 - 192.168.255.255 /16  --> This block everything in that range

Thanks all;
Carrzkiss
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Wayne Barron
Asked:
Wayne Barron
1 Solution
 
Ryan_RCommented:
The CIDR Notation is basically the xxx.xxx.xxx.xxx format that you are already using to enter in your IP addresses with the /xx suffix.

The following link should help you (from Wikipedia):
http://en.wikipedia.org/wiki/Subnetwork

I sincerly hope you understand it (even I had to read it more than once)

Ryan
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arnoldCommented:
no.

The CIDR notation translation is to take 8 out of the number.
Every time you take an 8 out the netmask
Netmask   = CIDR notation
255.0.0.0 = /8
255.255.0.0 = /16
255.255.255.0 = /24
255.255.255.255 = /32


/12 take out one 8
the remaining 4 = 128+64+32+16
255.240.0.0 = /12


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Wayne BarronAuthor Commented:
Ryan - I had already read the information on that site, and got some good information
But nothing that would explain in detail on what I am doing here.
----------
Arnold - Thanks for the information that you have provided, though it still does not tell me
What I was asking, I think you was just replying to what 'Ryan' had stated in his post.

---------
Once again:
If I add in

IP Address            Subnet Mask
172.16.0.0          172.31.255.255

Would that cover everything from
172.16 - 172.31.255.255
In the BLOCK list?
0
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lrmooreCommented:
>And then Enter   172.31.255.255   into our MASK field.
>Would that mean that it will look at and BLOCK out everything from    

No. 172.31.255.255 is a broadcast address, it is not a mask. You need a real mask, ie..

IP address 172.16.0.0  MASK 255.224.0.0
CIDR notation: 172.16.0.0/12
Would result in blocking all hosts in range 172.16.0.1 - 172.31.255.254

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Wayne BarronAuthor Commented:
Thank you lrmoore (And nice to see you again, been a while since we have crossed paths)
OK, so that would block everything if the subnet is:  255.224.0.0

Would that mean that this:

10.0.0.0 - 10.255.255.255    subnet: 255.255.255.255
Would that block everything from  10.0.0.0 ?

I have been admining our network here for years, and there is still a lot that I need to learn about it all.

Les say that you have the following Range of IP Address's

192.64.0.0   192.95.0.0
(&)
192.56.0.0 - 192.57.0.0
Would the bottom one be just
192.56.0.0    255.255.0.0
192.57.0.0    255.255.0.0

Thanks lrmoore for all the information that you can possibly provide me with on this.
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lrmooreCommented:
>10.0.0.0 - 10.255.255.255    subnet: 255.255.255.255
Would that block everything from  10.0.0.0 ?

Nope. that mask would block only 10.0.0.0
10.0.0.1 would get throuth
On the other hand,
10.0.0.0 - 10.255.255.255  subnet = 255.0.0.0
That would include the complete range

For 192.56.0.0 - 192.57.255.254 you can use one entry with mask 255.254.0.0
 192.56.0.0 / 15

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Wayne BarronAuthor Commented:
This is enough to drive someone nuts :)

lrmoore
Do you perhaps have a link that has/shows different IP Ranges that shows all this information on it?
This would make it a lot easier for myself and others to go to for information.
As, right now, I am totally lost, I am trying to follow, beleive me, but I am getting lost.

Because right now, we have 100's of entries in our BLOCK list, and are similuar to
192.56.0.0    255.255.0.0
192.57.0.0    255.255.0.0
&
192.64.0.0   192.95.0.0
& on & on.

Thanks
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lrmooreCommented:
You  might just want to get an IP subnet calculator
http://lantricks.com/lancalculator/
Easier to just work with the calculator than try to understand it all.
Took me a loooonnnnnggggg time to understand that subnet mask is nothing more than a binary string of 1's ... where the 1's stop, that determines the network ID and where the 0's start determines the host ID

255.0.0.0 = 11111111.00000000.00000000.00000000
255.255.0.0 = 11111111.11111111.00000000.00000000
255.255.255.0 = 11111111.11111111.11111111.00000000

255.255.254.0 = 11111111.11111111.11111110.00000000
<etc>

http://www.learntosubnet.com/

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Wayne BarronAuthor Commented:
That will do it.
Thanks once again for your helpful advice & information Mr. lrmoore;

Wayne
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Wayne BarronAuthor Commented:
lrmoore;
1 quick question, to make sure that I am going to do this correctly.

The following IP Address:
192.168.192/18

Wold the Subnet be:  255.255.192.0  ?

I am believing that is correct.
In the [LanCalculator] For the [Subnet Mask Info]
It list:
Class        Mask                Inv. Mask             Num Hosts           Prefix
C          255.255.192.0     0.0.63.255            16382                   /18

So, it looks like the correct MASK for the IP Address Range of:
192.168.192/18
Would be:  255.255.192.0

-------------
Also, in the list I just made above. The    
 Num Hosts   -  Is the the Number of IP Address in the  /18 ?

Thanks a bunch.
Wayne
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lrmooreCommented:
You got it!
Gets easier with a calculator, doesn't it?
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Wayne BarronAuthor Commented:
Thank you much buddy.
That little program is cool, it is really cool that the developer was nice enough to give it away for free. :)

Take Care and thank you once again, especially for the Calculator.

Wayne
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