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Explanation of subnets, for newbies
![[ca]](/s/countries/ca.gif) Shadow Hawkins on Friday, 18 April 2008 18:54:36
Hi,
I wanting to provide a small explanation of subnets, to a crowd who has no IPv6 experience and who are used to NATs for their home network. Could you please tell me whether this explanation below is correct. If you can improve on it, I would appreciate it. I will add the final explanation to the Wiki:
IPv6 has no notion of NAT, but it does have a notion of subnets, which are accessible from the outside world (if your router permits).
Basically your address gets assigned the first n bytes of the network address and you are free to chooses what you do with the remaining bytes. Typically what will happen is that the remaining bytes are worked out automatically based on the MAC address of the network interface. So, if you are assigned subnet prefix of
2001:4978:18a::/48
then you tell your router that you have
2001:4978:18a::/64
so that it assigns by default a subnet with id
2001:4978:18a:0000::/64
If you have a client host on that network which has the MAC address:
00:16:cb:9f:da:45
then you IPv6 address will become:
2001:4978:18a::216:cbff:fe9f:da45
where the last three bytes are the ID of you
Explanation of subnets, for newbies
![[us]](/s/countries/us.gif) Shadow Hawkins on Sunday, 20 April 2008 13:46:38
This explanation is helpful.
Could you explain the reasoning behind the assignment of the subnet portion. For example it is not clear why the address would not be 2001:4978:18a::16:cb9f:da45.
Why did you use the 216 and fe9f in the sequence?
Explanation of subnets, for newbies
![[nl]](/s/countries/nl.gif) Shadow Hawkins on Sunday, 20 April 2008 18:43:40
The interface identifier is in Modified EUI-64 format. In this case, it's based on the MAC-48 address:
first 3 bytes of mac address (with universal/local bit of first byte inverted) + ff + fe + last 3 bytes of mac address
MAC -> Modified EUI-64
00 -> 02
16 -> 16
cb -> cb
-- -> ff
-- -> fe
9f -> 9f
da -> da
45 -> 45
More information:
http://tools.ietf.org/html/rfc4291 (Section 2.5.1. Interface Identifiers; Appendix-A: Creating Modified EUI-64 Format Interface Identifiers)
http://en.wikipedia.org/wiki/MAC_address
Explanation of subnets, for newbies
Shadow Hawkins on Wednesday, 23 April 2008 22:17:31
I am not sure why the address is what it is. I would have thought the same as you, since this would match the MAC address, but this is not what I am seeing on my computer. In fact the address I provided is what I am getting on my subnet. If there is a good explanation then I would be curious to know it.
Explanation of subnets, for newbies
Shadow Hawkins on Wednesday, 23 April 2008 22:19:06
Oops, I missed the explanation provided.
Explanation of subnets, for newbies
Shadow Hawkins on Wednesday, 23 April 2008 22:49:54
I have added an entry to the Wiki:
https://noc.sixxs.net/wiki/Subnet
If you have any improvements to the entry, feel free to update it.
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