In order for your IPv6-only network to be able to reach IPv4 addresses, a gateway is required, which has access to IPv4. This is generally done with a NAT64/DNS64 gateway, which translates IPv4 addresses into "fake" IPv6 addresses, and accepts connections to those "fake" IPv6 addresses and translates the connections to the IPv4 network. These IPv6 addresses will be in the 64:ff9b::/96 range.