Assignable IP Addresses

[AGx-07_162]

I'm studying and trying to understand some concepts here. I have a sample question that I just can't seem to understand the answer to. Here's the question:

How many assignable IP addresses exist in the 172.16.1.10/27 network?
The answer appears to be 30.

What I'm not understanding [fully] is how this is calculated. From what I understand, the answer is all the addresses that fall between the subnet address and the directed broadcast address. What I think I'm doing wrong is misunderstanding what the subnet address actually is.

If someone could explain it to me, that would be great. My only other question in relation to this is whether or not the subnet mask I should be calculating the block size off of would be the classfull address for a 172 address (i.e. 255.255.0.0) or based on the prefix /27 (i.e. 255.255.255.224). My study material isn't very clear on this and online searching hasn't helped me much since almost every example I can find anywhere goes with a classful address rather than an extended one.

 

[Alabalcho]

2^(32-27)-2

Edit: Whoever wrote the question in this way does not understand IP networking. 172.16.1.10 cannot be the network address in /27 network.

Edit 2: Fixed nasty not-enough-coffee error

 

[kanewolf]

Maybe this will help -- http://unixwiz.net/techtips/netmask-ref.html

 

[boosted1g]

First of all thank you for asking detailed questions and not being like so many kids that are basically: here is my assignment, I have done no work on this, do all my work for me.

To do subnet math you have to understand binary.

Now we have no problem here helping you understand concepts but don't want to give you your answer, thus I will make the problem 192.168.1.0/25

With subneting you start from the last number and work to the beginning.
Each of the 4 octets being broken down into binary so 8 bits for the 192, 8 bits for the 168, 8 bits for the 1, and 8 bits for the 0 of the above listed IP.
So 4 octets x 8 bits per octet = 32 bits of binary data to represent said address. 255.0.0.0 would be /8 subnet, 255.255.0.0 is /16, 255.255.255.0 is /24, and then 255.255.255.255 would be /32, thus the subnet number is relating to the binary bit of data.
You can look at the chart here: http://www.digipro.com/Papers/IP_Subnetting.shtml
So for our problem of /25 you have 7 assignable bits (32-25 = 7). So counting the 0, 7 bits of data = 128 numbers. Now in networking the .255 and .0 address are strictly for subneting and are not assignable IPs. Thus 128-2 = 126 assignable IPs. And since you are removing only 128 addresses from the subnet and not the full 256, your final subnet is 255.255.255.128 (256-128)

 

[jfreggie2]

172.16.1.10 wouldn't be a network address, I would be an IP within the assignable IP address range. Google search for CIDR and you should get some help with classless routing. Also checkout this site http://www.subnet-calculator.com/cidr.php Makes it easy to stick in your information and it plops out network range/available address, etc

 

[boosted1g]

(32-27)^2-2

Edit: Whoever wrote the question in this way does not understand IP networking. 172.16.1.10 cannot be the network address in /27 network.

I was assuming the .10 was a typo and ment .0

How is that formula remotely correct though, just looking at a /24 which is 255.255.255.0 so 254 addresses.
32-24 = 8, 8^2 =64, 64-2 = 62.

 

[boosted1g]

.10 is not even possible with cidr anyways.

 

[Alabalcho]

(32-27)^2-2
How is that formula remotely correct though, just looking at a /24 which is 255.255.255.0 so 254 addresses.
32-24 = 8, 8^2 =64, 64-2 = 62.

Of course it is remotely correct (I still have to finish my coffee), I meant to write 2^(32-27)-2...

 

[AGx-07_162]

Thank you all for the information as this has helped me a lot. The question stumped me so hard I was starting to question whether or not it was even worded correctly. Again, I'm trying to understand the concepts. This was just a study question, not homework, so as I'm going through these I'm reviewing my notes and formulas at the same time trying make sense of what I think I know. This one just blew my mind. I DO think it was either poorly worded or just incorrect as I've come across a fair bit of information in this particular study guide that wasn't very clear. As an example, they would go over extending classful masks before explaining what prefix notation was...because that's not confusing.

Thanks again peoples. Back to studying.

 

[Kuixiv]

Eeesh, and I thought studying for the MTA was hard