[WorldMaker]

The double colon is a place where IPv6 addresses are easier to memorize than IPv4 addresses. Double colon just means "fill with zeroes". You don't have to remember how many zeroes to fill in. You don't have to type in a long string of zeroes. You just need to remember the prefix and suffix.

If you are assigning all the IPv6 IPs by hand like you might in a private network, the suffix can be as short as you wish: prefix:: is your Device 0, prefix::1 is your Device 1, prefix::2 is your Device 2, prefix::f is your Device 15, and so forth. At that point you only have to memorize your prefix, which is just your subnet. If the subnet you are assigned is 2001:db8:: for instance you just need to remember that 2001:db:: as your prefix and add whatever device number you need after it. Sure, it's unlikely to have a prefix exactly that short, but sometimes you can luck out with a lot of zeroes. Private networking today (Unique Local Addresses) is fd00::/8 where you are expected to pick a 40-bit random string to make it a /48. That's just three groups to memorize for your entire private network: fd01:2345:6789::.

::ffef from that perspective implies you are accessing Device 65,519. If you are trying to remember that many devices you may have other problems.

On the flipside, if you are assigning your own IPv6 suffixes anyway you can also use school kid "1337" "calculator" hacks in hex like setting up a key servers to be ::beef or ::dead or ::dead:beef or ::feed or ::8008. Potentially way more memorable "words" than numbers just between 0 and 255.