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I recently got a new cable modem and when I reset the IP it gave me an IPv6 IP address. This is a problem because it is a hassle to type in this long string of hexadecimal numbers and also PuTTY does not work with IPv6, at least that I have been able to figure out so far. Also, I do programming on the machine and it will be a hassle to support v6 addresses programmatically. By far the simplest thing would be for me to get an IPv4 address, but I cannot seem to do this. I tried:

sudo dhclient -4 -r eth0

But it did not work, I still got an IPv6 address. I am pretty sure that the ISP (Comcast) still supports IPv4. How can I force the DHCP server to give me an IPv4?

Note: this is a "dual stack" modem. When I bring up the modem administration page it does show an IPv4 "WAN IP", but when I try connecting to the ssh server using this IP the attempt to connect times out.

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    put a router between your network and the cable companies equipment, or contact the ISP for support. you could try disabling IPv6 and hope it negotiates a v4 address, but that is entirely determined by the ISP's policies applied to the router.
    – Frank Thomas
    Commented Jun 2, 2016 at 5:05
  • Are you talking about a link-local address (which starts with fe80)? Also even if it's global IPv6 address it is probably configured through SLAAC by the kernel.
    – Tom Yan
    Commented Jun 2, 2016 at 8:43

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You can't count on IPv4 being available everywhere all the time anymore. IPv4 addresses have run out and buying addresses on the market is expensive. More and more networks use technologies like NAT64 to give their users only IPv6 and provide central NAT services to be able to reach services that are only reachable over IPv4. That means that the device doesn't get an IPv4 address at all anymore.

Comcast uses IPv6 a lot these days, but I'm pretty sure they still offer some IPv4 connectivity. Possibly DS-Lite or MAP. You'll have to ask them what they support. But these protocols run on top of IPv6, so you'll always need IPv6. IPv6 is here to stay. In i.e. Belgium about half the internet connections already have IPv6.

And about the programming: you'll have to programmatically support IPv6 these days if you do any serious programming. Apple doesn't accept apps into the app store anymore if you don't, and many users like you will get IPv6. Dealing properly with IPv6 is one of the responsibilities of a good programmer now.

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