Forum - rtadv etc. Must subnet be 64 bits? :: SixXS

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 11:18:56

Damn, i just spent a long time formatting a long message with lots of info, and then when submitting, the forum asks "Who might you have been?" or something, because session is timed out :-( Note to self: Compose messages in notepad... Anyway: Is my trouble with hosts not getting an ipv6 in the subnet announced by rtadvd caused by having a prefixlen of 49?

rtadv etc. Must subnet be 64 bits?

Jeroen Massar SixXS Staff

on Friday, 16 July 2010 11:37:12

Router Advertised subnets must indeed be 64 bits, nothing more, nothing less. Which is why with a /48 you have 65536 /64 you can make ;)

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 12:04:38

Nice; its working now: hosts get a adress in my /64-subnet. Ip6's are auto-configured with prefix and mac, right? What happens if a host has a static ipv6, and another has a mac, that gives the same ip6?

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 12:05:03

Just "Don't do that, then"?

rtadv etc. Must subnet be 64 bits?

Jeroen Massar SixXS Staff

on Friday, 16 July 2010 12:28:52

Ip6's are auto-configured with prefix and mac, right?

The MAC address (EUI-48) gets converted into a EUI-64 address, then we have the last 64bits

What happens if a host has a static ipv6, and another has a mac, that gives the same ip6?

Duplicate Address Detection (DAD) handles this, but indeed, it might fail thus the last host not having an address. If you are going to assign static addresses, make sure you don't set the "Globally unique" bit of the EUI-64 address. See the picture at EUI-48 Address and the complete article about MAC Address for more about that.

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 13:20:57

> If you are going to assign static addresses, make sure you don't set the "Globally unique" bit of the EUI-64 address. Confused. "Set" = 0 or 1? 0: globally unique 1: locally administred According to the example, 06-00-00-00-00-01 is locally administred So ...:0600:0000:0001 is ok ...:0000:0000:0001 is not? Darn. Can't call router prefix::1 then?

rtadv etc. Must subnet be 64 bits?

Jeroen Massar SixXS Staff

on Friday, 16 July 2010 13:43:48

Those are bits, if you turn bit 0 on then it is globally unique, if you turn bit 1 on then it is locally administered. That is why Link Local addresses generally look like fe80::2xx:xxxx:xxxx:xxxx See the ipv6calc tool for more details, and of course the wikipedia article.

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 14:24:05

Those are bits, if you turn bit 0 on then it is globally unique, if you turn bit 1 on then it is locally administered.

I think you got it wrong here. If you turn bit 0 off, its unicast If you turn bit 0 on, its multicast If you turn bit 1 off, its globally unique If you turn bit 1 on, its locally administred.

Universally administered and locally administered addresses are distinguished by setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally administered. If it is 1, the address is locally administered. In the example address 06-00-00-00-00-01 the most significant byte is 06 (hex). The binary is 00000110 and the second least significant bit is 1. Therefore, it is a locally administered address.[4] The bit is 0 in all OUIs. If the least significant bit of the most significant byte is set to a 0, the packet is meant to reach only one receiving NIC. This is called unicast. If the least significant bit of the most significant byte is set to a 1, the packet is meant to be sent only once but still reach several NICs. This is called multicast.

My webservers etc should be public and globally unique, I think. So prefix::123 seems right to me. eg ipv6.google.com is 2a00:1450:8003::63

That is why Link Local addresses generally look like fe80::2xx:xxxx:xxxx:xxxx

Aren't there too many x'es? 48 bits=12 hex = 3 blocks fe80::2xx:xxxx:xxxx nginx.sixxs.net has IPv6 address 2001:7b8:3:4f:202:b3ff:fe46:bec b3 seems a locally controlled, multicast adress? b3 = 10110011. (I've been doing bitmaps since 1985 on Z80 :-) )

rtadv etc. Must subnet be 64 bits?

Jeroen Massar SixXS Staff

on Friday, 16 July 2010 14:44:55

You understand it wrong, see RFC4291 section 2.5.1 for the full explanation as per the RFC that defines it.

rtadv etc. Must subnet be 64 bits?

Shadow Hawkins on Friday, 16 July 2010 14:57:30

Oh. RFC4291:

Modified EUI-64 format interface identifiers are formed by inverting the "u" bit (universal/local bit in IEEE EUI-64 terminology) when forming the interface identifier from IEEE EUI-64 identifiers. In the resulting Modified EUI-64 format, the "u" bit is set to one (1) to indicate universal scope, and it is set to zero (0) to indicate local scope. The first three octets in binary of an IEEE EUI-64 identifier are as follows:

So I'm confused between IEEE 802 48-bit MAC and IEEE EUI-64 identifiers [EUI64] and 'inverting the "u"-bit' Anyway, I assume its ok to use this ip: 2001:16d8:dd5f::4

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