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How does Subnetting Work?

Is there a formula or a calculator for determining an IP address from CIDR that I was given: 38.103.17.160/27?

I would like to know the entire range of IP addresses from what I was given, but I have no idea how to break it down.

I am far, far from being a networking person so please go easy on me!

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marked as duplicate by Mark Henderson Sep 26 '11 at 0:26

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
I am most likely never going to become a networking admin or need the level of complexity that the possible duplicate answer, or even the serverfault.com/questions/12854/cidr-for-dummies answer provides. I sell domain names and am having to look into some things on my server. I just want to verify what someone else is telling me... If someday I am required to learn this stuff for my job, then I will certainly reference those. –  ONDEV Sep 26 '11 at 1:06
1  
The basics of IP addresses and CIDR are just that - basics. Very, very basics. This was correctly closed. –  Ward Sep 26 '11 at 3:30
    
If the explanations are too complex for you why not use one of the 10 gazillion subnet calculators on the Internet? –  John Gardeniers Sep 27 '11 at 5:27

3 Answers 3

up vote 0 down vote accepted

Yes. That little number is supposed to give you that very information, but does require a bit of calculating. Have a look at this article which explains subnetting and how to calculate how many hosts/subnets that /27 refers to:

How does Subnetting Work?

And once you know how it works, you can use some calculators to make work easier :)

  • http://www.subnetmask.info/
  • ipcalc - a tool in *nix systems
  • there are hundreds more with a simple Google search for 'subnet calculator'

EDIT: Jason mentioned my favourite one that I had forgotten about: A Way To Determine IP Range From CIDR?

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Thank you for these calculators!! –  ONDEV Sep 26 '11 at 1:51

http://www.subnet-calculator.com/ is awesome for those (like me) that don't like to do the math themselves.

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Where do I plug in that "/27" on that calculator? Would that be the "Mask Bits"? If I do that then the range is 38.103.17.161 - 38.103.17.190 which doesnt include the IP 38.103.17.160 or is that range correct? And I will take a look at the duplicate too - sorry folks. –  ONDEV Sep 26 '11 at 0:44
    
That's correct. 38.103.17.160 is the network id, it's not a usable address. –  Jason Berg Sep 26 '11 at 0:55
    
Thanks so much for this, it is a godsend! –  ONDEV Sep 26 '11 at 1:04
    
Ah, yes. There it is. I hadn't used it for a while so I didn't remember the address for this tool. I prefer it out of all the online based calculators I've seen. –  uzzi09 Sep 27 '11 at 4:44

I generally use a tool called sipcalc (http://www.routemeister.net/projects/sipcalc/) to get a quick answer on subnetting questions, though you can also go through and do the binary math to figure it out.

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Thank you for this also, I wish I could upvote it... –  ONDEV Sep 26 '11 at 1:50

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