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The background to my question is a recent Cisco networking skills exam, where one requirement was to set up an ISP router with a static route back to the company's border router. I used a Class B /16 route whereas it was marked as wrong because they specified that the NAT pool allocated to the company was and therefore should be a Class C /24 static route. I was under the impression that ISPs do no use classless routing and will route the entire IP address block (which for 183 is Class B) to the company's border router.

Which is correct, at least in terms of the rules/theory?

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Classful networking is long dead. Nobody uses it for anything. – Zoredache Nov 8 '12 at 8:23

Classful routing isn't used anywhere on the Internet anymore. And even if it was, it is networks that are classful, not subnets. So the subnet could still be a /24 even if it was a subnet of a /16 classful network.

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