Is the ipv6 address 2001: 1d5: : 30a: : 1 a valid address? why or why not?

Not a valid IPv6 address A valid IPv6 address consist of 8 groups of 4 hexadecimal numbers separated by colons ":". But that can make for a rather long address of 39 characters. So you're allowed to abbreviate an IPv6 address by getting rid of superfluous zeros. The superfluous zeros are leading zeros in each group of 4 digits, but you have to leave at least one digit in each group. The final elimination of 1 or more groups of all zeros is to use a double colon "::" to replace one or more groups of all zeros. But you can only do that once. Otherwise, it results in an ambiguous IP address. For the example of 2001:1d5::30a::1, there are two such omissions, meaning that the address can be any of 2001:1d5:0:30a:0:0:0:1 2001:1d5:0:0:30a:0:0:1 2001:1d5:0:0:0:30a:0:1 And since you can't determine which it is, it's not a valid IP address.