IPv6 information

Im 90% clear now but there is still one bit I cant make sense of.
My understanding is:

All external addresses must start with 2001: or 3001: (until in the future they may use 4001:)

the /3 part means that you need to look at the first 3 bits in the ipv6 address. so:

2001 in binary = 0010 0000 0000 0001
so you look at the first 3 bits of this binary value (001)
this explains where you said there are 8 ::/3 blocks as you cannot use the bit value of 1 because you can only use the first 3 bits, so you can only go up in values of 2.

but you mention that all external addresses must start with a 2 or a 3? how can it start with 3 if you cant use the bit value of 1?

thanks

::/1 would split into two blocks

all the values that start with zero on the one hand and all the blocks that start with one on the other.
eg 0000-7fff and 8000 to ffff

::/2 would split into four blocks
00, 01, 10 & 11
eg 0000::/2, 4000::/2, 8000::/3 & C000::/3

::/3 is
000, 001, 010, 011, 100, 101, 110, 111

the text i am reading is :
the address prefix for currently assigned global addresses is 2000::/3. Therefore, all global unicast addresses begin with either a 2 or a 3.

So i take this to meen all external IPv6 addresses must start with 2001: or 3001:

but as you mentioned the ::/3 is
000, 001, 010, 011, 100, 101, 110, 111

this equates to
0,2,4,6,8,10,12,14

so how can it start with 3001: when the ::/3 cannot create a 3?

thanks

0,2,4,6,8,10,12,14 are the start of each block
in hex that is
0,2,4,6,8,A,C,E

The block start with those numbers
so ::/2 would be
0,4,8,C
that is
0-3, 4-7, 8-B, C-F

or for ::/3
0-1,2-3,4-5,6-7,8-9,A-B,C-D,E-F

>The block start with those numbers
>so ::/2 would be
>0,4,8,C

This bit I understand fine. /2 means you can only use the first 2 binary digits so you can only go up in 4's (4 outcomes), /3 means you can only use the first 3 binary digits so you can only go up in 2's (8 outcomes).
When you convert this to hex, numbers 10-16 become letters so
/2 = 0,4,8,C
/3 = 0,2,4,6,8,A,C,E

This is the bit I don't understand

>that is
>0-3, 4-7, 8-B, C-F

can you please explain the maths behind this as the "that is" didn't really explain it?
you said it gives you 0-3,4-7,8-B,C-F, why and how does it give you this? those hex values cover all 16 possible values?

thanks

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