Cannot connect to host even though it is reachable

Question

There are 4 machines in my network, connected via a consumer router with default config (also acting as DCHP server).

All machines can ping all other machines with both IPv4 and IPv6, except one (let's call it) A cannot ping one other (B) on IPv6. That is to say:

ping B -4
Pinging B [192.168.1.101] with 32 bytes of data:
Reply from 192.168.1.101: bytes=32 time=2ms TTL=128

ping B -6
Ping request could not find host B. Please check the name and try again.

ping B
Ping request could not find host B. Please check the name and try again.

The other machines can be pinged fine from A, and everyone can ping everyone else (4 and 6), except for A pinging B with IPv6. A uses Windows 10, B uses Windows 7, all well updated. The two others use Windows 10 and 7.

I've rebooted all machines, done ipconfig /renew and /flushdns and a couple obscure netsh reset commands, without any change in (non-)connectivity. The HOSTS file does not contain any related name.

I'd think the router is not the culprit, as it uses default settings and almost everything passes through fine. I'm at a loss trying to figure out what could lead to this scenario.

Output of ipconfig /all on A:

Windows IP Configuration

Host Name . . . . . . . . . . . . : (A)
Primary Dns Suffix  . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Onboard LAN:

Connection-specific DNS Suffix  . :
Description . . . . . . . . . . . : Intel(R) Ethernet Connection (2) I219-V
Physical Address. . . . . . . . . : ...E8-6D
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::f9e5:219c:34f6:5ca0%13(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.207(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 18 October, 2018 14:24:30
Lease Expires . . . . . . . . . . : 19 October, 2018 14:24:29
Default Gateway . . . . . . . . . : 192.168.1.1
DHCP Server . . . . . . . . . . . : 192.168.1.1
DHCPv6 IAID . . . . . . . . . . . : 154180643
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-22-EF-A4-B7-30-9C-23-9A-E8-6D
DNS Servers . . . . . . . . . . . : 1.1.1.1
                                   192.168.1.1
NetBIOS over Tcpip. . . . . . . . : Enabled

At B:

Windows IP Configuration

   Host Name . . . . . . . . . . . . : B
   Primary Dns Suffix  . . . . . . . :
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No

Wireless LAN adapter Wireless Network Connection 5:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft Virtual WiFi Miniport Adapter #4
   Physical Address. . . . . . . . . : xxx
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes

Wireless LAN adapter Wireless Network Connection 4:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft Virtual WiFi Miniport Adapter #3
   Physical Address. . . . . . . . . : xxx
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes

Wireless LAN adapter Wireless Network Connection:

   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Intel(R) Centrino(R) Advanced-N 6235
   Physical Address. . . . . . . . . : ...BE-08
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   Link-local IPv6 Address . . . . . : fe80::a9e1:931b:f386:77c0%12(Preferred)
   IPv4 Address. . . . . . . . . . . : 192.168.1.101(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Lease Obtained. . . . . . . . . . : Thursday, October 18, 2018 13:23:31
   Lease Expires . . . . . . . . . . : Friday, October 19, 2018 14:54:15
   Default Gateway . . . . . . . . . : 192.168.1.1
   DHCP Server . . . . . . . . . . . : 192.168.1.1
   DHCPv6 IAID . . . . . . . . . . . : 213169782
   DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-1A-DC-90-A3-B4-B6-76-9D-BE-08
   DNS Servers . . . . . . . . . . . : 1.1.1.1
                                       192.168.1.1
   NetBIOS over Tcpip. . . . . . . . : Enabled

Ethernet adapter Local Area Connection 2:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : TAP-Windows Adapter V9
   Physical Address. . . . . . . . . : ....8D-B4
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes

Ethernet adapter Local Area Connection:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Realtek PCIe GBE Family Controller
   Physical Address. . . . . . . . . : ...B1-28
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes

Ethernet adapter Bluetooth Network Connection:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Bluetooth Device (Personal Area Network)
   Physical Address. . . . . . . . . : ...BE-0C
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{BFA18DB4-A58C-4422-A0F4-DBA8D03C6BCF}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #2
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{A3D0404D-292D-462C-A42A-601FD738E85F}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #3
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter Teredo Tunneling Pseudo-Interface:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{9A3C139D-E540-4E21-9C7F-7FC6C60587F5}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #5
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{28C98D1F-037A-4B36-8B3A-5DB1B1110349}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #7
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter 6TO4 Adapter:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft 6to4 Adapter #2
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{1CEA6C11-E4FC-4DC2-A31A-9190E66A2DE2}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #6
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{01356168-2149-4366-9D26-1CC9774596AF}:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft ISATAP Adapter #8
   Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
   DHCP Enabled. . . . . . . . . . . : No
   Autoconfiguration Enabled . . . . : Yes

All IPv4 and IPv6 settings on all devices are set to automatic, i.e. using DHCP.

Answer 1

You should make sure that Teredo, 6to4 and ISATAP are disabled on both machines. Having any of these enabled but unconfigured can make IPv6 not work properly. And your output shows that all three are enabled and unconfigured.

Microsoft recommends against using ISATAP (in Understanding IPv6, Third Edition, p. 302) and says that native IPv6 should be used instead. Public 6to4 service is officially deprecated (RFC 7526; BCP 196). And Teredo has a long list of issues (only one IPv6 address which is guessable, limited NAT traversal support, various connectivity problems) which recommend against its use generally.

Unfortunately Windows had a bad habit of enabling all of these by default.

If you aren't knowingly using IPv6 transition technologies, you should explicitly disable them.

Right-click on Command Prompt and choose Run as Administrator. Then paste in the following commands:

netsh int ipv6 isatap set state disabled
netsh int ipv6 6to4 set state disabled
netsh int teredo set state disabled

You can also disable these with PowerShell (again, as Administrator):

Set-NetIsatapConfiguration -State Disabled
Set-Net6to4Configuration -State Disabled
Set-NetTeredoConfiguration -Type Disabled   

Then restart your computer.