No WAN IP address from Spectrum
I swapped out the cable modem at my house with a new one with phone support. After the new modem was installed, no IP address was supplied by Spectrum. I decided to check DHCP by plugging my Windows 10 laptop into the modem. DHCP worked. Hmm. So I tried my Mint 18 laptop - no address. I then installed wireshark on both and checked the DHCP process. Sure enough Windows discover was immediately followed by a DHCP offer. No response on the linux box. Hmm. Then I noticed that the Windows system also got a reply from a DHCPv6 server and IPv6 was enabled on the interface.
Back to the interface definition and added IPv6 to the WAN port. Success. Now I'm on both IPv4 and IPv6 on this port.