Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

IPv6 address questions

I'm having trouble understanding IPv6 addresses. In IPv4 the CIDR slash notation means what subnet the address is in. But a /48 in IPv6 does not mean subnet. I have no idea what it means. For instance I've seen the address 2001:0:1:5::1/64, and I have no idea what the /64 means. Can someone explain it?

Link local address: it's not enough to put an FE80 to identify this kind of address, but for some reason, they decided to put FFFE in the middle of the ipv6 address. What were they thinking? Why do they need to identify this kind of address TWICE within the address, AND why couldn't they put the FFFE at the end or the beginning, but instead they put it right in the middle? WTF?

3 types of addresses? What why? with that many bits in an address there's no reason to have 3 of them. Theres enough for everyone.

Illogical allocation of bits. A global ip address reserves the last 64 bits for the host ip. This is equivalent to 18 quintillion hosts on ONE SUBNET! YEAH RIGHT! There's no way that's even remotely logical, feasible, or practical. The more I look at IPv6 notation, the more I think my 5th grader could have come up with a better design.

Also, why didn't they adopt the OSI model of addressing, like in ISIS. That has more than enough addresses for everyone.

Obviously i'm missing the point completely on IPv6. It sounds like the most unthoughtful pile of rubbish ever conceived. So can someone please direct me to a place or explain what the thought process was when creating this new addressing scheme?

bonus points: what happened to ipv5, and what was ipv1,ipv2,ipv3? I think the inventor should have called it IPv2000, because he really went overboard.


Re: IPv6 address questions

Hi Richee,

ISPs generally give ipv6 addresses to companies with /48 prefix length.

The company receiving this can create its own subnets within the /48 and /64 range.

The last 64 for bits of the address are generally used to insert the mac-address of the local interface when using stateless auto-configuration, but you can use it for subnetting as well, if you configure your addresses manually.

For the other questions I think this link could give you more info:

And one more little addition:

Nothing is perfect in this universe. Everything can be considered good or bad, it is just a question of viewpoint.

Try to look at ipv6 from both viewpoints. You will have surprises.