Networking – Setting up IPv6 on a router behind another router


I have a TP-Link Archer C2 AC750 (V1) router. It works well for IPv4 but I am not able to setup IPv6.

Network topology

The discussed router shares internet connection to devices in my flat (Macs and iPhones of various age). The WAN port of the router is connected to the house router which shares a common DSL connection (the ISP is O2 Czech Republic) across the building using Ethernet.

Direct connection

I can connect one of my Macs directly to the wired house network. It gets all needed information for both IPv4 and IPv6 connection. The webpage opened in Safari shows my public IPv6 address (beginning with 2a00:, so no 6to4 tunnel).

Full listing follows. I am hiding my MAC and IPv6 addresses for privacy reason.

$ ifconfig en0
    ether xx:xx:xx:xx:xx:xx 
    inet6 fe80::xxxx:xxxx:xxxx:xxxx%en0 prefixlen 64 scopeid 0x4 
    inet netmask 0xffffff00 broadcast
    inet6 2a00:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx prefixlen 64 autoconf 
    inet6 2a00:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx prefixlen 64 autoconf temporary 
    nd6 options=1<PERFORMNUD>
    media: autoselect (100baseTX <half-duplex>)
    status: active

$ cat /etc/resolv.conf | grep -v '^#'
domain .
nameserver fe80::1

Current router settings

The IPv6 configuration of my router follows. I am hiding the IP address for privacy reason. The first half of the address (64-bit prefix) is still the same.

IPv6 Status

Connection Type: Dynamic IPv6
Connection Status: Connected
IPv6 Address: 2a00:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx /64
IPv6 Default Gateway: fe80::0000:0000:0000:0001
Primary IPv6 DNS: ::
Secondary IPv6 DNS: ::
- IPv6 LAN
IPv6 Address Type: RADVD
Prefix Length: 64
IPv6 Address: N/A


IPv6 WAN settings

When I change the Addressing Type to the only other option, DHCPv6, the router never gets an IPv6 address.


IPv6 LAN settings (RADVD)


IPv6 LAN settings (DHCPv6)

Regardless of whether I choose RADVD or DHCPv6 with Delegated prefix, my computer never gets an IPv6 address. My computer gets some IPv6 address with DHCPv6 and Static prefix but it still cannot connect the IPv6 internet. It cannot even ping either router.

Can anyone help me with this settings? Or, since some people report that the IPv6 support of TP-Link routers is incomplete (e.g., should I rather switch to OpenWrt?


Unfortunately, later I realized that OpenWrt is not yet ready for my router, see OpenWrt Wiki and OpenWrt Discussion for details.

Since @Michael Hampton suspected the upstream (house) router to be misbehaving, I am showing its current IPv6 LAN Auto Configuration settings.

IPv6 LAN Auto Configuration of the house router

I tried to disable M-bit custom mode but no change…


The IPv6 information in the house router is following. Again, the IP address is hidden for privacy reason.

IPv6 enable/disabled:   Enabled
IPv6 Primary DNS Server:    2a00:1028:1:910::1
IPv6 Secondary DNS Server:  2a00:1028:1:911::1
Active IPv6 Prefix:  
Active IPv6 Prefix Length:   
LAN interface Link-Local address:   fe80::0:0:0:1
Manual configured prefix:    
WAN interface Link-Local address:   fe80::xxxx:xxxx:xxxx:xxxx
WAN interface User Setting Global address:  2A00:1028:xxxx:xxxx:0:0:0:1/64
IPv6 DefaultGateway:    fe80::xxxx:xxxx:xxxx:xxxx
LAN IPv6 Address:    
Default IPv6 interface Gateway:  

Best Answer

It turns out that the house router gets the range of IP addresses with the prefix /64 which seems to be not divisible into multiple subnets.

An IPv6 address is eight groupings of numbers:

  • Network address - the first three groupings of numbers (first 48 bits) in the subnet mask
  • Subnet address - the fourth grouping of numbers (the 49th through 64th bits) in the subnet mask
  • Device address - the last four groupings of numbers (the last 64 bits) in the subnet mask

Source: IBM Knowledge Center – Subnet masks (IPv4) and prefixes (IPv6)

The ISP seems to be not following the Recommendations on IPv6 Address Allocations to Sites.

In particular, it recommends the assignment of /48 in the general case, /64 when it is known that one and only one subnet is needed and /128 when it is absolutely known that one and only one device is connecting.

Further reading


I got IPv6 working by setting up the router as a switch. However, this means that all the devices and traffic on the subordinate (flat) network are exposed to the upstream (house) network.


  1. Manually set the LAN IPv4 address of the subordinate router. You will need it to access the configuration of that router, at least the parts that will be applicable after the transition, for example Wi-fi.
    • If you can influence the upstream router configuration, reserve a static IPv4 address in the configuration of the upstream router.
    • Otherwise set an address from a different range, for example if the upstream router supplies 10.0.x.x addresses, use 192.168.x.x. You’ll have to configure your computer to use the same range whenever you need to access the router configuration.
  2. Turn off the DHCP server (both IPv4 and IPv6) of the subordinate router.
  3. Disconnect the upstream cable from the WAN port and connect it to a LAN port.

Based on: (in Czech)