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I have seen IPv6 subnets described as /64 or /128, particularly by hosting companies. Does a /64 subnet mean there are 64 ipv6 addresses? Or 64bits worth of ipv6 addresses?

marathon
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  • possible duplicate of [What do the numbers used in a IPV6 address mean?](http://superuser.com/questions/379795/what-do-the-numbers-used-in-a-ipv6-address-mean) – Ƭᴇcʜιᴇ007 Jun 10 '14 at 19:50
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    I disagree with the duplicate candidate. That one appears to ask about an IPv6 address itself. This question asks about the IPv6 CIDR notation. @techie007 – user Jun 10 '14 at 20:11
  • @MichaelKjörling The accepted answer on the marked possible dupe explains _all_ the numbers involved in an IPv6 address, including the slash notation. The OP of the dupe (in comments) even mentions /64 and admits he doesn't know what that means either. But hey, takes more than just my vote. ;) – Ƭᴇcʜιᴇ007 Jun 10 '14 at 20:26
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    @techie007 Good point made. Well, the arguments have been made; now I suppose it's up to the community how to vote. After all, the auto-comment even does say *possible* duplicate. – user Jun 10 '14 at 20:48
  • I do not think this should be marked as a duplicate. When doing a search for "IPv6 slash notation", this result is in the top 10 Google search for me. On the other hand, a huge number of results pop up for the general IPv6 specification, and it would take a lot longer to dig through these answers to find this specific sub-question answered. I think highly more specific questions should not be closed even when much more general questions exist with an answer that includes an answer to the much more specific question. This is a matter of saving significant search time. – Dan Nissenbaum Oct 09 '15 at 04:53

1 Answers1

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It means the number of bits in the network part of the address. If you compare with IPv4 there are 32 bits in an IP, of which any number can be assigned to the network part. So for example, if you have a /24, you have 8 bits left for the host part of the address (2^8=256).

If you extend this example to IPv6, which is 128 bits, for a /64 you would have 2^64= 1.8446744e+19 addresses available within the subnet. Also, a /64 is considered a 'normal' network in IPv6 terms (so end user LAN's would be /64's).

mtak
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    It follows that in IPv6, an address specified to /128 is a single host address. – user Jun 10 '14 at 20:13
  • I was going to post an answer, but you said pretty much what I intended to say before I had a chance to. :) – user Jun 10 '14 at 20:17