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I have a class C that I want to better understand before I implement a change.

Right now,

ip address
ip address secondary

So, where is the address in that mix? should be the broadcast for the first one, and should be the cisco secondary ip.

And ...

How then do I add a /28 ( right before this segment:

ip address secondary

In fact, I'll probably want to add it as a subinterface

Can anyone help me to better understand whats going on and then how to do it?

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up vote 4 down vote accepted

First of all, you need to read this:

How does Subnetting Work?

Your first subnet-definition has addresses from You already knew that.

Your second subnet-definition has addresses from, with a broadcast of Yes, the definition says .65 in it, but once you've read Evan's excellent article you'll know why the second range starts at .64.

I'm not sure what you're asking for regarding a "/28 right before this segment". The next /28 would be defined:

ip address secondary

And would give a range of .80-.95

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