- In this video we're going to take a look at IPv6 address notation. So here we have an IPv6 address. And as you may notice it uses the hexadecimal number system. Zero through nine, A through F. And, if we look at this address every one of those digits is a hexadecimal digit, so if we take a look at these digits each digit is actually one hexadecimal digit representing four bits. So that first digit two is in binary 0010. This gives us our 128-bit address, but we'll, represented it using hexadecimal values. So representing an IPv6 address in hexadecimal we break it up into eight 16-bit segments. So each one of these segments is four hexadecimal digits, each digit as we saw it is four bits. So each one of these four hexadecimal segments sometimes called a hextet, or we just call it a segment. Each one of these segments is 16 bits. So we have eight of these segments, eight times 16 gives us 128 bits. So that looks like it's a lot of digits to have to write. Well we have two rules to help us reduce the size of writing or representing an IPv6 address. The first rule is omitting any leading zeros in any one of these segments. For example, this first address we can omit in each one of these segments where we have a leading zero or leading zeros, we can omit those zeros. So we can omit the leading zeros. Lets take a look at another example. We have leading zeros, those can be omitted. So wherever we see a double colon, and in this case a single digit A, we know there's three zeros omitted. Let's take a look at this last example. Once again, any leading zeros in any of the 16-bit segments can be omitted, they do not have to be written. So it's important to know that this rule can only be applied to leading zeros not trailing zeros. For example, if we allowed trailing zeros to also be omitted, we wouldn't know whether those were leading zeros, trailing zeros, or a combination of both being omitted. It would lead to ambiguity. This case the only real option is leading zeros, so when we see AB we have two leading zeros being omitted. The second rule is that any single contiguous string of two or more 16-bit segments consisting of all zeros can be represented by a single double colon. For example, notice here that we have several zeros in our address. In this segment here the leading zero can be omitted. In this segment here the leading zero can be omitted. But we have a contiguous string of two or more 16-bit segments right here. That can be represented by a single double colon. So we can write this address as 2001, db8, omitting the leading zero, double colon which is representing all these zeros here. Five contiguous 16-bit segments of all zeros. That can be represented by the double colon. So using these two rules helps us reduce the size of representing an IPv6 address.