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Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

cgrey8
Aspirant

How is a home/private network's ipv6 network number assigned to Cable Modems?

For typical IPv4 home network deployments, the private network is made up of private IP addresses (usually 192.168.0.0/24). All devices share the router's public (Internet-routable) IP address assigned to it by the ISP via the cable-modem serving as a NAT-router. This serves as both a firewalling protection of the devices on the private network as well as alleviating the need for each device on your home network having a highly-valued IPv4 Internet-valid IP. And for most client devices, this works just fine.

 

However IPv6 expands the available number of networks and IPs by such a great number that each person on the planet could have more IPs assigned to them than they could possibly use. So NAT-routing isn't really necessary. And to protect the devices, traditional firewalling techniques can be used instead of relying on the devices not being internet-accessible as a form of security.

 

Now to my point, ISPs issue the cable modem an IPv6 address just like they issue IPv4 addresses. But how do they get assigned the home network IPv6 network number? I understand that IPv6 devices can generate their own host number (lower 64bits of an IPv6 address), but they still need to be told what their network number is (the upper 64-bits). This is normally done via SLAAC or DHCPv6. But the cable modem needs to know what network number to give when devices on the local network ask. 

 

So, how does the ISP commuincate, to my cable modem, the network number it wants my cable modem issue to my home network IPv6 devices? I'm assuming it needs to issue this so that its routing tables can update to know that return-packets destined to my IPv6 devices must get routed to my cable-modem's IP...unless my Netgear C6300 is NATing IPv6 exactly like it does IPv4.

 

The IPv6 IPs my cable modem is assigning have a prefix of 2601: (Comcast/Xfinity). When I go to websites that tell me my IP address, they correctly identify both my router's public IPv4 address but they know my device's actual IPv6 address (or at least the IPv6 address being used to communicate with the website) suggesting that the IPv6 IP my device is using is Internet-accessible, and thus isn't being NATed the way my IPv4 communications are.

 

In traditional commercial router deployments, each router is configured to know the network numbers of its various datalinks. But in the case of residential cable-modems, I'm ASSUMING the cable-modem is told this information. I'm just curious how that's accomplished and if it's a standard networking protocol or something specific to DOCSIS devices. Or perhaps is the cable modem assigned an IPv6 network from the factory and communicates that to the ISP for them to update their routing tables. I can see a number of possibilities and I can't seem to find any answers google-searching. It's evidently not something most people care to know.

Model: C6300|AC1750 Cable Modem Router Docsis 3.0
Message 1 of 9
FURRYe38
Guru

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Message 2 of 9
cgrey8
Aspirant

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

I've read through this entire Wikipedia page hoping maybe there was something here that explained what I wanted to know. And there isn't. 

 

There was a section talking about ISPs, but nothing that detailed how home IPv6 network numbers are assigned to IPS customers. There was verbiage that did indicate that Comcast does only issue a single /64 network as opposed to a range of networks, which only confirms what I know so far. 

 

But this still doesn't answer my question. How are cable modems told what their subordinate network's network number is? The subordinate network, in this case, is the LAN that your personal devices would be connected to.

Message 3 of 9
FURRYe38
Guru

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Message 4 of 9
vkdelta
NETGEAR Expert

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

There is no NAT in IPv6. It was designed to avoid that in first place.

 

Basically IPv6 in cable has three parts.

 

you Cable Gateway (2-in-1 combo box) has 3 key items:

1. CM IPv6 address --> Single IPv6 address to manage the Cable Modem via SNMP. You will most likely not able to see this. 

2. WAN IPv6 address -> Single IPv6 address pointing to your device's WAN port. This is same stack as your WAN Public IPv4

3. Delegated IPv6 prefix -> your ISP is supposed to send a whole 64 bit (or lower sizee) delegated prefix for your home devices. they get divided and shared between all the devices.This prefix is sent over by ISP and not dictated by the device. 

 

all the traffic from Delegated Prefix subnet is routed via WAN IPv6 address (but NOT NAT'ed)

 

when your device is talking to a website, it is actually usings its own IPv6 address.

 

here is old comcast presentation from John B

 

https://archive.nanog.org/meetings/nanog46/presentations/Tuesday/Brzozowski_introDHCP_N46.pdf

 

additionally look at Ipv6 section below 

 

https://archive.nanog.org/meetings/nanog46/presentations/Sunday/Byju_Intro_DOCSIS_N46.pdf

 

 

Message 5 of 9
vkdelta
NETGEAR Expert

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Message 6 of 9
FURRYe38
Guru

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Thank you Vkdelta.

Message 7 of 9
cgrey8
Aspirant

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Some of that was useful. I need to watch the videos, but I think I found what I was ultimately looking for is referred to (and mentioned above) is "prefix delegation." It appears the DHCPv6 protocol and services is what performs Prefix Delegation.

https://en.wikipedia.org/wiki/Prefix_delegation

 

This was enlightening to me since I wasn't aware that DHCP was what did this.

 

Thanks for the help...

Message 8 of 9
FURRYe38
Guru

Re: How is a home/private network's ipv6 network number assigned to Cable Modems?

Smiley Wink

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