Quick IP subnetting question

The Spyder

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Joined
Jun 18, 2002
Messages
2,628
Guys, I am horrible at IP subnetting. I actually am planning on taking a course on it here in the near future. So your help in this matter is greatly appreciated.

I am working with a client on some basic IP validation testing. On their last email they asked me to scan the ip of xx.xx.xx.xxx/28. Now I understand /28 is a bank of 16 available networks with 14 hosts per network and a max of 224 total usable hosts.
But what does this mean for me? I am just a bit lost here.... :confused:
 
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whrswoldo

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Joined
Jun 6, 2002
Messages
3,138
Guys, I am horrible at IP subnetting. I actually am planning on taking a course on it here in the near future. So your help in this matter is greatly appreciated.

I am working with a client on some basic IP validation testing. On their last email they asked me to scan the ip of xx.xx.xx.xxx/28. Now I understand /28 is a bank of 16 available networks with 14 hosts per network and a max of 224 total usable hosts.
But what does this mean for me? I am just a bit lost here.... :confused:

First you need to determine the subnet mask. The /28 means the first 28 digits are on or 1:

11111111:11111111:11111111:11110000

Now you need to convert from binary to get the subnet mask:

255.255.255.240

Now to determine the block size take 256 and subtract the last # of the mask:

256 - 240 = 16

That means your valid subnets are x.x.x.0, x.x.x.16, x.x.x.32, etc. The first number in each subnet is reserved for the subnetting and the last number is reserved for broadcast, so your valid IPs are going to be x.x.x.1-x.x.x.14 for the first subnet, x.x.x.17-x.x.x.130 for the second subnet, etc.
 

berky

2[H]4U
Joined
Aug 28, 2001
Messages
2,233
ugh, I remember learning subnetting for the first time. I was so confused just as you are. When I finally realized that the whole "16 available networks in a /28" is complete BS, I was better able to understand subnetting. First, by stating this, you are assuming that you are breaking up a /24 network. that isn't always the case. let's learn by example:

suppose that my company was assigned network 4.4.0.0/16 by my ISP.

If I need a /28 for some network, that doesn't mean i have to break up my entire network into all /28's!!!
I can break up my network however I want, as long as they follow the binary rules.

so, for example, I could do the following:

4.4.0.0/28
4.4.0.16/29
4.4.0.24/29
4.4.0.32/27
4.4.0.64/26
4.4.0.128/25
4.4.1.0/24
4.4.2.0/23
4.4.4.0/22
4.4.8.0/21
(remaining unallocated)

so, it should be obvious that you can break up any subnet as you wish, but they must follow the proper binary limitations
 
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