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IPv6 information

Posted on 2012-09-19
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Hi,

I am trying to get my head around IPv6 and wanted to know what 2000::/3 is? Have been reading but there aren't any good explanations. Also what /3 bit is?
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Question by:RMGS
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Accepted Solution

Ashok Dewan earned 672 total points
ID: 38413064
I have recently hit my head on many sites. But when i came to this site

http://ciscoiseasy.blogspot.com
Then read the guide of IPv6 from this site then my head became cool. I hope you like his tutorial on IPv6

For information this 2000::0/3 is for public network addresses. It is not for LAN like private address in ipv4 10.0.0.0/172.16.0.0/192.168.0.0. It is for public your ISP will give you one public address which start from 20xx::xx/3. 3 bits have been used here.
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bevhost earned 1328 total points
ID: 38413319
In IPV4 you may have seen 0.0.0.0/0 as the default route or the everything route
Problem is that it includes routes like 10.0.0.0/8 and 192.168.0.0/16 which aren't meant to be routed to the Internet.

so you could route ::/0 to the internet and that would work, but as it happens, so far and for a good while to come all routeable IPv6 Addresses are in the range 2000:: to 3fff::

In HEX to Binary...
0 is 0000
1 is 0001
2 is 0010
3 is 0011
4 is 0100
5 is 0101

so to get the ones the start with 2 or 3, then you need the ones that start with these three bits 001

There are 8 ::/3 blocks

0000::/3, 2000::/3, 4000::/3, 6000::/3, 8000::/3, A000::/3, C000::/3, E000::/3

One day in a long time from now they might start allocating addresses beginning with 4 and you also need a 4000::/3 route.
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Author Comment

ID: 38413683
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
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Expert Comment

ID: 38415632
::/1 would split into two blocks

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
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Author Comment

ID: 38417561
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.

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
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Expert Comment

ID: 38419947
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

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
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Author Comment

ID: 38420894
>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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Assisted Solution

bevhost earned 1328 total points
ID: 38585619
These digits are the start of each block
>so ::/2 would be
>0,4,8,C
>that is
>0-3, 4-7, 8-B, C-F
so in the block that begins with 0 it can begin with any number up to the start of the next block.  so the first block is numbers starting with 0, 1, 2 or 3, the next block has numbers starting with 4,5,6 or 7 and then 8,9,A,B and so on.
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Expert Comment

ID: 38615656
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