dual stack question

I seen a recent post asking how to code a dual stack that is capable of handling both IPv4 and 6. So now I'm curious! So what is a dual stack exactly? Maybe ill code one, if its not to complicated. I did Google it and the answers were very vague. Also has IPv6 already succeeded IPv4? How important is it to know how to work with IPv6 in the field?

Basically, a socket is dual stack if it can bind to both an IPv4 address and an IPv6 address. You do this by creating an IPv6 socket then setting the IPV6_V6ONLY socket option (under the IPPROTO_IPV6 level) to 0.

This is supported in Linux 2.4.21 and later kernels and Windows Vista and later.

Daul stack guide ---> long.ccaba.upc.es/long/045Guidelines/eva/ipv6.html