Izzy H.

asked • 05/16/19

Why is the fourth IPv6 address invalid?

Q) Which of the following IPv6 addresses is invalid?

  1. 21E5:69AA:FFFF:1:E100:B691:1285:F56E
  2. ::
  3. 59FB::1005:CC57:6571
  4. 56FE::2159:5BBC::6594

[Q2bi 9608/11 Oct/Nov 2018]

According to the answer sheet, all the IP addresses are valid except the last.

I know that the first is standard IPv6 address with a "shortcut" of writing ":0001:" as just ":1:"

And the second is an Ipv4 address in IPV6 format

However I don't understand the last two. If I'm counting correctly, the third has only 12 bytes when "expanded" (59FB:0000:0000:1005:CC57:6571) but IPv6 addresses should have 16 bytes. While on the other hand the fourth does indeed have 16 bytes. If what I've said is correct, shouldn't the third be the invalid IP address while the fourth be the valid one? Instead of the other way round?

An explanation would be greatly appreciated!

Many thanks in advance.

1 Expert Answer


David W. answered • 05/16/19

4.7 (80)


Izzy H.

I've just researched on how to shorten IPv6 addresses, am i right to say that the whole "::" can only be used once in the entire address and "::" doesn't necessarily mean "0000:0000" and can sometimes be longer?


David W.

Research to answer your question is a much better way to learn than to listen to "lectures" (no matter how interesting). You can use the double colon notation only once in any given address. The two colons tell the operating system that everything in between them is a zero. So, would you ever see three colons in a row?


Izzy H.

Got it. I'll keep the tip of researching answers in mind for the future as I'm just starting to realize that my AS/A Levels textbook I'm studying from is not that good in explaining things, on top of that I've no background in studying CS... So apologies for my lack of understanding. And about that triple colon in an address, the third colon is to separate the address "::" from its port number... Right???? Not quite sure how reliable the answers I'm getting are (https://superuser.com/questions/515379/what-is-the-difference-between-and-0-0-0-0-from-the-netstat-an-output)


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