Selecting a network prefix - Study/Exam help

I'm looking for help on a practice question regarding network.
Am wondering someone could dumb down the answer for me and explanation because I'm really trying to get a better understanding of this.

The question goes:

Your company has a main office that contains 255 client computers.
The client computers are located on a subnet that uses the network ID of 10.10.1.0/24.

The company plans to open two branch offices. The offices will be configured as follows:

Branch1: 100 client computers
Branch2: 50 client computers

You need to select a network prefix for each office to ensure that there are enough IPv4 addresses for each client computer.

The solution must minimize the number of unused IP addresses.

Which network prefixes should you select?




The available choices are:

/24
/25
/26
/27
/28
garryshapeAsked:
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Joseph DalyConnect With a Mentor Commented:
I dont really like this question because it seems like bad info.

The main office has 255 clients and only a /24 subnet. This is incorrect because with a /24 you have only a maximum usable IPs of 254 not 255.

For the 50 person office the probably want you to say /26 since that will give you 62 usable addresses and the 100 office you would want to use a /25 since it gives you 126 usable addresses.

In a real life situation for these two remote offices you would probably just use a /24 on each of them for future growth.
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Kent FichtnerInformation Technology Systems SupervisorCommented:
I Agree with Joe.  If your question is what does /24 mean and why then you would want to look up subnetting.  it is a topic that can drive you nuts.

Basically if you think of a IPv4 address in binary, then it is made up of 8  ones or zeros per period

10.10.1.0 would then be 00001010.00001010.00000001.00000000

The slash notation is where (starting from the left) is left as static.

In the case above /24 wouldn't allow for the first 24 numbers to change, and only the last 8 can.  Since 11111111 in binary is 255 (-1 for broadcast) you can get a max of 254 per octet.  To answer your question just take the number of users, convert to binary, and see how many numbers it would take to make it:

100 clients is written as 01100100.  Since that the largest digit is in the 64's place, you can static everything to the left.  In this case, as Joe stated above, that would be 25 digits or /25 notation.

Hope that helps a little.
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garryshapeAuthor Commented:
Thanks, how do you calculate usable addresses based on the /#?
Do you have to convert to binarY?
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Kent FichtnerConnect With a Mentor Information Technology Systems SupervisorCommented:
The best idea is to convert to binary.  That way you can count the number for the slash notation.

Cisco has a good way of looking at it.

I would type out the answer, but subnetting is a long topic, not too difficult once you get it, but takes a while to understand.
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garryshapeAuthor Commented:
ok thanks, yeah this is for a microsoft windows exam which seems complicated. Half the exam is IPv4 and IPv6 stuff. Better take my networking exam at the same time I guess.
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