What makes a network IPv6 capable

To declare a network is capable of IPv6 connectivity, there are certain infrastructural components that have to be in place before hand.

Operating System: The operating system (or the client) has to be IPv6 enabled client. This could be in the form of dual stack client or IPv6 only client so that it would have IPv6 address assigned to it.

Dynamic Host Configuration Protocol (DHCP6) server: Now a days, almost every network is managed by DHCP to assign IP address, though this is not a mandatory device. But to declare that the network is IPv6 enabled, there has to be a DHCP6 server if the network needs to be managed in a stateful manner to autoconfigure the network IP addresses. The alternate is stateless autoconfiguration that doesn’t need DHCP6 server (which comes with some security risks). In that case no DHCP6 server is required.

Router: the router has to be able to recognize and process IPv6 packets. Other option is tunneling which doesn’t say that the network is IPv6 enable but just a work around

Domain Name System: the DNS has to be capable to resolve IPv6 addresses so that the source host can reach out the destination IPv6 hosts

There are few other types of devices that are sometime placed in the network like NAT, Proxy server, Firewall. If these devices are present in the network (and probably are), then all these devices should also be IPv6 aware so that the end to end connectivity can take place. 

There’s a hard way to make the IPv4 network works for IPv6 communication, which is through tunneling. In that way the IPv6 packets are transmitted by encapsulating into IPv4 packets. But this is a complicated way to achieve IPv6 connectivity with high cost of configuration and performance