The loopback addresses (both in IPv4 and IPv6) is an address which represents the same interface of a computer. Whenever we communicate to a loopback address (both in IPv4 and IPv6) the TCP/IP protocol stack will loop the packets back on the same interface, without even leaving the interface. The loopback addresses are typically used by programmers for development and testing of network applications without even having network configurations.
In IPv4, we have an entire network (127.0.0.0/8) reserved as loopback addresses. But almost all leading operating systems use the name "localhost" to represent an IPv4 loopback address 127.0.0.1. Rest of the addresses in 127.0.0.0/8 are not widely used.
In IPv6, the IPv6 address reserved for loopback use is 0000:0000:0000:0000:0000:0000:0000:0001/128. This loopback address is so lengthy and can be further simplified as ::1/128. Click the below links to learn more about how to simplify and shorten an IPv6 address.
Unspecified address in IPv6 is the IPv6 address with all binary bits set to "0". In IPv4, the IPv4 address used as Unspecified address is 0.0.0.0/0. Unspecified address is used by an Operating System, before an IPv4 address or IPv6 address is configured on it. (Example, the Operating System is configured to obtain an IP address dynamically using DHCP, and the machine is in the process of obtaining an IP address from the DHCP server)
IPv4 and IPv6 routers will not forward packets with the unspecified address.
In IPv6, the unspecified IPv6 address is 0000:0000:0000:0000:0000:0000:0000:0000/0. This unspecified address is so lengthy and can be further simplified as ::/0. Click the below links to learn more about how to simplify and shorten an IPv6 address. Following table shows IPv4 and IPv6 loopback and unspecified address