Internet Protocol Version 6
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IPv6 Prefixes and Comcast Business

Why does Comcast give out /56 prefixes when /64 is what is standard via the IETF?  Comcast is actually giving me 8 more address bits for a total of 72 for me.  Comcast has eaten into their own subnet address space by doing this.  It is only creating confusion and will eventually have to be fixed (at whose cost?).  I have the ability to address 4.72 x 10^21 hosts (a bit much)......  Your Cisco gateway/NAT/firewall/whatever is confused in this process (it's not a router).


Are those extra 8 bits for me to allocate via my routers (if I were a large company?)?  If so, how?

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Official Employee

Re: IPv6 Prefixes and Comcast Business

Thanks for reaching out to our Comcast Business, caugusti!


I found this forum that has a pretty awesome explanation on how Comast provides it's IPv4's and IPv6 addresses. https://forums.businesshelp.comcast.com/t5/IPV6/Summary-status-of-Comcast-IPv6/td-p/35071.


If this doesn't clear it up for you, send me a private message with your full name, address and, phone number on the account. I'll take a look at your network and try to give you a more detailed breakdown of how your network is delegating the IPv6 prefix.

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New Contributor

Re: IPv6 Prefixes and Comcast Business

The IETF does not say that a single /64 is "standard" for a site (even a home site), but rather that a /64 is the *smallest* prefix which should ever be used in IPv6. One of the reasons for the /64 minimum is to facilitate stateless autoconfiguration (SLAAC). 


In RFC 6177, the IETF explicitly recommends that even home sites be given "significantly more than a single /64," although they do back off on their earlier recommendation that every site be given a /48 by default. In the same document, they call out /56 as being a reasonable compromise prefix length that should satisfy most home sites. 


The main reason that even a small business might want a larger prefix than a /64 has nothing to do with the number of IP addresses they need. Instead, because of the recommendation that IPv6 subnets not be divided smaller than /64 (which would break SLAAC, possibly among other things), you need a bigger-than-/64 prefix in order to correctly segment your IPv6 network into multiple subnets. This is an important capability for many users, even small business users. 

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Official Employee

Re: IPv6 Prefixes and Comcast Business

The IPv6 prefix assigned to the LAN of the Business IP Gateway, is typically a /64 even though your gateway is allocated a /56. More information on using the IPv6 addresses can be found through the following links. 





If you have any further questions or concerns please reach out through our forums at any time. 

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