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New problem solver


20 Messages

Tuesday, April 11th, 2017 12:00 PM

Do I have static IPv6 addresses?

For the first time, I logged into my Comcast Business account, went to the "Manage Services", "Internet", page, and noticed a link to "View Static IP", so I clicked on it.

It brings up a page that not only shows the static IPv4 addresses I know very well, but also "Static IPv6 Information", which is a "/56" address that starts with "2603:3024:".

Are these address really static? Can I depend on them not changing when our modem loses power?

And, also important, will Comcast make a PTR record with these addresses?

Accepted Solution

New problem solver


18 Messages

6 years ago

Yes and no.  The static IPv6 is a prefix delegation and if you have an IPv6 compliant router it can accept that prefix delegation and subsequently assign or allow you to assign static IPv6 address it devices attached to your network.


The caveat here is that your router is at the mercy of your cable modem in terms of which subnet(s) are assigned to it.  As of this writing, you cannot go and pick a subnet from your prefix delegation and assign it statically.  The cable modem must do the assignment.  


This means that you have kind of a hybrid, yes the prefix delegation won't be taken from you and changed, but it is possible for the cable modem to change which subnet has been assigned to your router.   It hasn't happened to me...yet...but because there is no provision in the cable modem to set up a static IPv6 route from your cable modem to your router, you can't pick your IPv6 subnet.


I imagine that once they allow us to specify IPv6 static routes on the cable modem (like they allow for IPv4) then you can pick your own subnet...until have to let your router accept the assignment automatically from the cable modem.


'Been round and round with Comcast on this one.




New problem solver


37 Messages

@flyingrobots​ I know this post is 5 years old, but it seems my modem (Cisco DPC3941B) with DOCSIS version and Software image DPC3941B_4.12p24s1_PROD_sey I'm able to choose a /59 out of my /56 with the proper setup and static ipv4 and static ipv6 block.

First, the modem:

Gateway -> Connection -> Local IP setup -> ipv6 -> Lan ipv6 address assignment -> [X] Stateful(Use Dhcp Server)

Gateway -> Connection -> Local IP setup -> ipv6 -> Lan ipv6 address assignment -> DHCPv6 Lease Time [ Forever ]

Next on your (personal) router you'll need to enable ipv6 prefix delegation.

Set prefix delegation hint to ::/59 and you will get a random /59 from within the /56.  However, I've tried specifying an exact /59 and I was able to get that one assigned, routed & working.

I don't know if this worked when your original answer was posted, but this is working for me at the end of 2022.