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How does Subnetting Work?

Amazon's elastic compute tool (among others) requires the ip block format for their command:

ec2-authorize websrv -P tcp -p 80 -s

I may be doing this wrong, but as far as I can tell I need to use the block format even for a single IP address. 1) So, how would I do that for this IP?

Several years ago I took a CCNA class, and I remember going over IPs and subnets, masks, broadcast addresses, class a/b/c networks, etc. However a lot seems to have changed since then - for example I don't think you can tell what "class" a network is in just by looking at it anymore - sometimes they could be multiple classes.

2) Anyhow, my second question is where do I go to get a refresher on all these things?

3) Or should I just be using ipcalc or an online calculator to do it all for me - and if so, which one?

marked as duplicate by Mark Henderson Nov 22 '11 at 2:07

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.


1) For the single IP, you can say "".

2) and 3) You can google for CIDR. You will see the wikipedia page on CIDR, which should have something about subnetting to refresh your memor, and should have a calculator pretty high up in search results.


I use the following in a pinch...



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