IPv6 Address Planning

Chapter 1. Where We’ve Been, Where We’re Going

Introduction

I see it, but I don’t believe it.

— Georg Cantor

The standard LAN interface assignment in IPv6 is a /64. Or, to be more explicit, a subnet with 64 bits set aside for network identification and 64 bits reserved for host addresses. (If this is unfamiliar to you, don’t fret. We’ll review the basic rules of IPv6 addressing in the second chapter.) As it happens, 64 bits of host addressing makes for a pretty large decimal value:

18,446,744,073,709,551,616

It’s cumbersome to represent such large values, so let’s use scientific notation instead (and round the value down, too):

That’s a pretty big number (around 18 quintillion) and in IPv6, if we’re following the rules, we stick it on a single LAN interface. Here’s what that might look like in common router configuration syntax (Cisco IOS, in this case):

!
interface FastEthernet0/0
 ipv6 address 2001:db8:a:1::1/64

Now if someone were to ask you to configure the same interface for IPv4, what is the first question you’d ask? Most likely some variation of the following:

“How many hosts are on the LAN segment I’m configuring the interface for?”

The reason we ask this question is that we don’t want to use more IPv4 addresses than we need to. In most situations, the answer would help determine the ...