Comcast Participates in ISOC IPv6 Panel Discussion

Today at the 77th meeting of the IETF, Comcast is participating in a panel discussion organized by the Internet Society (ISOC). The panel is focused on the increasing momentum behind global IPv6 deployments. To listen to a live webcast of the event, please visit this ISOC web page. You can check out our slides here. Discussion will be led by ISOC's Chief Internet Technology Officer, Leslie Daigle, and panelists include Geoff Huston, from APNIC, Jason Livingood, from Comcast, and David Temkin, from Netflix.

General Trials Preparation Update

We have been extremely pleased with the positive reaction to our IPv6 trials, and especially that over 5,000 customers have volunteered to participate from all around the U.S. The map below gives you a sense for where those volunteers are located, and you can see that this represents geographically diverse interest in IPv6. We're using the location of volunteers to inform our decision-making on where different IPv6 trials will be conducted. You can expect more from us in the next few weeks.

Comcast and ISC Donate IPv6 Open Source Software

As noted here in our blog, Comcast and the Internet Systems Consortium (ISC) have now released free open source software that may help facilitate the industry's transition to IPv6. We are releasing Address Family Transition Router (AFTR) software, which you can find now on ISC's website here. We believe AFTR software is a critical component of Dual-Stack Lite technology. This milestone represents the culmination of almost five years of joint IPv6 research and development by Comcast and ISC.

IPv6 Trial Plans for 2010 Announced

Comcast has been a leader in IPv6 development for many years. Our leadership continues today with the announcement of a plan to conduct real, production-network trials of IPv6 technology this year. We encourage customers to sign up now to volunteer to participate in one of our trials.

The transition from IPv4 addresses to IPv6 addresses is a necessity, as the available pool of IPv4 addresses will at some point be exhausted for all Internet users. These trials will help Comcast to identify and solve any areas of difficulty involved in the transition to IPv6, and to determine what approach will be the easiest and most seamless to our customers. Comcast will continue to share what we learn with the Internet community, particularly with the IETF, for the benefit of other users of the Internet.

IPv6 Trials - Timing and FAQs


Trial #1 will evaluate tunneling IPv6 over IPv4, using "6RD" technology. 6RD is an open Internet standard developed in the IETF's Softwires working group. This trial enables us to explore how we may be able to provide production level IPv6 service to customers who have only been issued IPv4 addresses by the ISP network.

Trial #2 will evaluate a native Dual-Stack IPv6 deployment. We will test issuing both IPv4 and IPv6 addresses to the customer premise. This trial will attempt to suit a variety of customer premise configurations, and will evolve over time as the customer premise equipment (CPE) support for IPv6 matures. Native, dual-stack is central to our IPv6 strategy and we expect that the native dual-stack solution will be a significant part of the IPv6 transition, enabling IPv6 technology to evolve globally while still being able to provide seamless services to the traditional IPv4 Internet.


Trial #3 will evaluate tunneling IPv4 over IPv6, using "Dual-Stack Lite" technology (aka DS-Lite). DS-Lite is an open standard defined in the IETF's Softwires working group. DS-Lite will involve provisioning only an IPv6 address at the customer premise, and creating a IPv4 over IPv6 tunnel between the customer's home gateway and an Address Family Transition Router (AFTR) in our network. That AFTR enables sharing IPv4 addresses among DS-Lite IPv6 customers. DS-Lite can be seen as the mirror image of 6RD; 6RD enables IPv6 service to IPv4 customers, whereas DS-Lite enables IPv4 service to IPv6 customers. DS-Lite offers the ability to continue an IPv4 service when IPv4 addresses are no longer readily available or are otherwise constrained.

Trial #4 will evaluate how to deploy IPv6 to our Business Class customers, on a native Dual-Stack IPv6 basis.

Note: All dates are tentative and are subject to change. Also, trial dates have been set conservatively, which means that some trials may begin earlier than indicated above. As each trial begins, this site will be updated.

For detailed FAQs regarding these trials, please see our IPv6 Trial FAQs.

IPv6 Deployment FAQs

In November 2011, Comcast announced that we started our first pilot market deployment of IPv6. This follows technical trials started in 2010 and over 6 years of IPv6 development work. We are the first major ISP in North America to launch IPv6 to residential Internet users. This is the first phase of launch for us, with subsequent phases later in 2011 and throughout 2012.

What is this?

We are announcing our plan to begin deploying IPv6 to our XFINITY INTERNET customers. Initially this is in a limited area of California (Pleasanton), but will soon expand to other areas of our network that have been part of our IPv6 technical trials.

Will IPv4 be disabled?

No. We are continuing to provide an IPv4 address to each customer and are now additionally providing IPv6 addresses; this approach is referred to as Native Dual Stack. IPv4 will remain as-is while IPv6 is introduced. Based on our testing and industry best practices, this model will offer the greatest flexibility and seamlessness during the IPv6 transition.

How will you enable support for IPv6 in this first phase?

In our first phase of deployment, we will enable IPv6 on selected standalone computers. This is the case when a customer has just one computer, and where that computer is plugged directly into a cable modem. We will begin with a small number of DOCSIS 3.0 cable modem models (see those modems noted as IPv6-ready at, which will expand over time. However, Comcast does not directly enable IPv6 functionality on end user systems as this depends upon the capabilities of that computer's operating system. The current major consumer operating systems capable of this are Windows 7, Windows Vista, and Mac OS X 10.7 (Lion).

Can I opt out of IPv6?

No. However, you can disable IPv6 on your system (see the next FAQ).

Can I disable IPv6?

Yes, IPv6 can be disabled on your computer. Please refer to your operating system documentation for instructions on how to do this. Please note that operating system vendors typically do not recommended disabling IPv6.

With IPv6 for standalone computers, what size IPv6 prefix will I receive?

We will allocate a single IPv6 address (/128), since we know that only a single device is connecting, with no additional need to sub-net. We plan to continue to assess address allocation policies as we deploy, particularly given how very new IPv6 is from an operational standpoint globally.

When will Comcast support IPv6 for home networking?

Comcast is planning to begin pilot market deployment of IPv6 home networking support in the near future.

Are you using tunneling or Large Scale NAT (LSN)?

No, not at this time. We are using Native Dual Stack, which means a customer gets both IPv6 and IPv4 addresses simultaneously. As a result no tunneling or Network Address Translation (NAT) is necessary. We believe this approach offers meaningful performance benefits to our customers compared to the alternatives.

What comes next?

This is the first phase of our IPv6 deployment. As with any pilot market deployment of new technology, it is possible that a technical issue may arise which causes us to delay our next steps or even to temporarily disable IPv6 in existing pilot markets. Assuming no issues are encountered, we will expand to additional CMTSs in our network. This will most likely happen first in those areas where we have already been conducting IPv6 trials. At the same time, we will be working to expand the number of eligible cable modem models that can support IPv6. In a subsequent phase we will enable customers with home gateway devices to use IPv6, but we are not yet prepared to commit to a date for doing so since this will, in part, depend upon how the first phase of our pilot market deployment proceeds.

Do your DNS servers support IPv6?

Yes. Our DNS servers, both authoritative and caching, support IPv6 today in our network. The servers can send and receive queries for IPv6 records (AAAA) and can communicate over IPv6 to end-users. The IPv6 addresses for our DNS recursive resolvers (which customers use for DNS lookups) are 2001:558:FEED::1 and 2001:558:FEED::2.

My IPv6 capable version of Windows that is directly connected to an IPv6 capable Comcast cable modem is not being assigned an IPv6 address.

It is possible that your computer or device operating system was created in such a way that some critical system information was duplicated via the computer manufacturer. This information is essential to how Comcast provisions IPv6 capable devices. Computers or devices where this information was incorrectly duplicated will be unable to complete the IPv6 provisioning process required to acquire an IPv6 address and configuration information. This is also known as DHCPv6, or Dynamic Host Configuration Protocol for IPv6. The information that is duplicated is called DHCPv6 DHCP Unique Identifier (DUID) and is intended to uniquely identify individual computer systems or devices. Please refer to RFC3315 ( for detailed technical information. Microsoft has published information that provides an explanation for this issue as well:

Microsoft has released updates in November 2012 for Windows 7 and Windows 8 systems, which automatically detects and corrects this condition. By default, these updates should already have been installed on your computer.

The following provides more details:
Windows 7 –

Windows 8 –

Does Tomato USB work with Comcast Native IPv6?

In order to use Tomato USB firmware with Comcast’s native IPv6 implementation, a few changes will be required.  There is currently a bug in the IPv6-capable builds from Toastman and Shibby that adds an erroneous default route for IPv6.

Enable IPv6

In order to use IPv6 with Comcast’s network, you need to enable the correct mode.  Go to Basics -> IPv6 in the left panel.  Then, selecet DHCPv6 with Prefix Delegation as the IPv6 Service Type.  Make sure Enable Router Advertisements and Accept RA from WAN are both checked.  Click Save.

Add custom script workaround

The workaround for the IPv6 default route bug involves adding a custom script into the WAN Up section.  More information is available at  if you would like to read the details.  Thank you to koitsu from DSLReports for this fix.
Go to Administration -> Scripts from the left menu.  Select the WAN Up tab.  Copy and paste the following directly into the text box:


# Workaround for TomatoUSB bug where a spurious default IPv6 route is

# added for no justified reason, resulting in packets getting forwarded

# effectively to /dev/null.


# 1. Temporarily disable accepting IPv6 RAs on the WAN interface.  This

#    will stop the kernel from automatically adding a default IPv6 route

#    when such an RA is received via the WAN.

# 2. Delete ALL default IPv6 routes.  In effect this deletes the spurious

#    IPv6 default route, as well as any default IPv6 routes received via RA.

#    Sadly the "ip" command does not give you a way to differentiate between

#    the two, since the one we truly want to delete lacks "proto kernel".

# 3. Restore honouring IPv6 RAs via the WAN.  Within 60-120 seconds (often

#    within seconds on Comcast) a default IPv6 route should be added by the

#    kernel.  You can use "ip -6 route show default dev `nvram get wan_iface`"

#    to verify; you should have only one route ("default via fe80::xxx ...").




echo 0 > /proc/sys/net/ipv6/conf/`nvram get wan_iface`/accept_ra

ip -6 route flush default dev `nvram get wan_iface`

echo 2 > /proc/sys/net/ipv6/conf/`nvram get wan_iface`/accept_ra 


Click Save.  Go to Restart on the left panel and restart the device.