IPv6 and passthrough mode
I just spoke with a Comcast technical representative and they clarified that Comcast currently does not offer IPv6 addresses when a customer modem is in pass through mode as it requires DHCP enabled and managed on the modem and that passthrough is the demarcation line so it's on the customer. It would be helpful to know what others have done in this situation. For example, did others just add the additional bridge layer and how much effort that was or they found another way. As an aside, currently there is a block of 5 static IPv4 addresses. I appreciate any of your thoughts. Thanks.
New problem solver
4 months ago
I have the same setup as you. You will need to take your cable mode out of bridge/passthrough mode, and your "router" behind the modem has to do DHCPv6-PD and send a "hint". If your "router" doesn't support dhcpv6-pd functionality, you'll need to connect devices directly to the cable modem to have functional ipv6.
If it does have dhcpv6-pd functionality and you have an ipv6 /56 static allocation (you can see this in the business portal, on the static ip information), you can send a hint of /59 and you will get a block out of that. there are 8 /59s in a /56 block.
If you don't care if the /59s change (eg: you dont have any servers that need static ips):
You can specify the prefix hint as ::/59
If you decide you want a specific block
Instead of your "prefix hint" being ::/59, try specifying one!
There is an online calculator here that I found: https://subnettingpractice.com/ipv6-subnet-calculator.html
Take your /56 address from the comcast portal and put it in the "ipv6 address field" such as 2001:db8::/56
Then next to the left calculate button, put "8"
You get something like this (Generated from that webpage):
You can use any of those as the "prefix hint", eg: "2001:db8:0:e0::/59" Please replace the above values with what the page gives you for your specific /56.
THis works for me with a Mikrotik router. I haven't yet tested opnsense. Once this is working for you, there is going to be a router specific way to assign other interfacers an "offset". Inside a /59 there are 32 /64s ("networks") you can assign to different interfaces on your router. If you are using a more simple router (such as something yoiu might buy at best buy) you probably only need one of these /64 networks. If that is the case, set your offset to "1" and you are (hopefully) done.
If you have multiple networks or vlans, you can assign offsets from 0-31. Be aware that if you use the first /59 generated from that webpage, offset 0 is probably in use already (at least it is on mine) and is non assignable (it wont work). If you use the 2nd-8th /59 generated you can use offset 0.
If you let us know what specific router you have & what the exact options are, I'm happy to help further.