Why “why” matters

A participant on the NANOG mailing list recently made the comment that “with a small amount of conceptual knowledge, the differences between IPv4 and IPv6 become very very small.” True! And perhaps an anecdote will illustrate why conceptual knowledge, not just practical know-how, is sometimes important.

This is a true story: At a previous employer, a local admin had over 250 computers on his network, and wanted to add more. He needed more IP addresses, so we extended his /24 to a /23. It was all coordinated – after work on a Friday, and he was going to renumber everything. This was before DHCP had been fully deployed on our site, and he had a lot of statically configured machines.

He rang the next day in a bit of a flap. “The new addresses
don’t work!”

We pressed for more info.

“They all work fine up to 255”, he said, “but from 256 up they aren’t even accepted by the configuration utility! I’ve tried all the way up to 300!”

He wasn’t dumb – far from it – he’d just never been outside a /24 before, so had never needed to understand what the numbers meant.

When it comes to moving from IPv4 to IPv6, a lot of knowledge is directly portable – provided it is based on a conceptual understanding of how IP addresses work. Purely practical know-how – short-cuts, tricks and memorised combinations – is much less portable, and may even obstruct understanding.

That’s why in my IPv6 courses, we start with a look at how addresses actually work, what they mean, and how the bits and bytes fit together. For those that know already, it’s a good refresher. But for surprisingly many, it is a door not only to an deeper understanding of IPv6, but also to a much better understanding of IPv4.


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