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Today's Topics:
1. Re: Wrong prefix length is advertised (Simon Hobson)
2. Re: Wrong prefix length is advertised (Sten Carlsen)
----------------------------------------------------------------------
Message: 1
Date: Thu, 24 Mar 2016 13:50:08 +0000
From: Simon Hobson <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: Re: Wrong prefix length is advertised
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Juergen Bachsteffel <[email protected]> wrote:
> subnet6 fda0:65da:e55a:1::/64 {
> range6 fda0:65da:e55a:1::0010 fda0:65da:e55a:1::0100;
> }
>
>
> My problem is. The clients (Windows 7 and Linux) get a IPv6-Address, but
> when I ping between them, nothing happens.
>
> Reason:
>
> If if type 'ip -6 addr' on Linux, I can see that the prefix length is
> /128. Same effect on Windows.
>
> e.g fda0:65da:e55a:1::20/128
What do "ip -f inet6 route" and "ip -f inet6 neigh" show ?
At the risk of starting off the whole "discussion" again, IPv6 needs some of
the "old rules" to be un-learned - especially in terms of what constitutes a
neighbour ! You might want to sit down with a fresh cup of tea while you try
and get your head round it - it took me a while and after having been in the
"that's nuts" camp, it suddenly turned into one of those "ah, that makes sense"
moments.
In IPv4, who is a "neighbour" is intimately linked with subnets and subnet
masks (prefix lengths). So if your address is 192.168.1.57/24 and my address is
192.168.1.123/24, then we are neighbours (assuming we are on the same physical
network of course).
Of course, you may also have the address 192.168.1.69/24 - so now we have
duplicate information stored.
And we are used to every IP address having an associated subnet mask.
I don't know if you've ever worked with a multi-subnet network (where, usually
because one got filled up, there are two IP subnets sharing one "bit of wire"),
but if you have then you'll know that for hosts in the two subnets to
communicate then they either have to be told explicitly about the "other"
subnet(s), or they have to communicate via a router.
With IPv6 things are different. IP addresses are distinctly separated from
routing and the concept of who is a neighbour. An IP address does not have a
prefix length, it is just an address. But, because of the way the network code
(at least in the Linux world) has been built with this intimate linking of IP
address and subnet mask - the IPv6 code "needs" a prefix length to be specified
so that the value can be filled in in the internal data structures. In
principle, whatever value is supplied, /128 should be used internally.
You're probably, like I was, thinking this is nuts - so how does a host
determine what else is on the network ?
This is where routing advertisements come into play. A router will advertise to
nodes a number of key information. There are some flags (such as whether nodes
should auto-configure or look to something else such as a DHCP server for
addressing), but one key thing is a list of prefixes used on the local network
that should be considered local.
So, taking your example address above, the host has obtained the address
fda0:65da:e55a:1::20 (ignore the /128, that's just there because of this
historical baggage in the code). That tells it nothing about what hosts it can
talk to.
The router should have sent out an advertisement stating that the prefix
fda0:65da:e55a:1::/64 is in use AND that hosts can use neighbour discovery to
find others in this subnet. It should be immediately obvious at this point that
the router could advertise more than one prefix, and then hosts could
automatically find and talk directly to neighbours in the other prefix(es).
Why this emphasis on whether neighbour discovery can be used by the host ?
For those brought up with broadcast networks like ethernet then it's probably a
good question. There are some network types which from an addressing POV look
the same, but don't support "broadcast" between different clients - AIUI many
radio systems are like this. So if the host fda0:65da:e55a:1::20 wants to talk
to fda0:65da:e55a:1::21 it cannot do so directly. So even though
fda0:65da:e55a:1::20 and fda0:65da:e55a:1::21 are in the same prefix, they
cannot directly talk to each other - instead they must send the packets up to
the router.
So while in the IPv4 world we (without configuration fudges) have only two
combinations of same|different subnet and neighbours|not-neighbours. In the
IPv6 world we have all 4 combinations possible - as well as the two obvious
combination we can have hosts that are in the same prefix but are not
neighbours, and hosts that are in different prefixes which are neighbours.
Hope that helps a bit !
------------------------------
Message: 2
Date: Thu, 24 Mar 2016 18:21:22 +0100
From: Sten Carlsen <[email protected]>
To: [email protected]
Subject: Re: Wrong prefix length is advertised
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"
Just to point out that while in IP4 the DHCP gives all info needed, in
IP6 both DHCP and RADV are needed to have the full picture.
It is between the lines in Simon's description below, but is one point
that took me some time to realise.
On 24/03/2016 14:50, Simon Hobson wrote:
> Juergen Bachsteffel <[email protected]> wrote:
>
>> subnet6 fda0:65da:e55a:1::/64 {
>> range6 fda0:65da:e55a:1::0010 fda0:65da:e55a:1::0100;
>> }
>>
>>
>> My problem is. The clients (Windows 7 and Linux) get a IPv6-Address, but
>> when I ping between them, nothing happens.
>>
>> Reason:
>>
>> If if type 'ip -6 addr' on Linux, I can see that the prefix length is
>> /128. Same effect on Windows.
>>
>> e.g fda0:65da:e55a:1::20/128
> What do "ip -f inet6 route" and "ip -f inet6 neigh" show ?
>
> At the risk of starting off the whole "discussion" again, IPv6 needs some of
> the "old rules" to be un-learned - especially in terms of what constitutes a
> neighbour ! You might want to sit down with a fresh cup of tea while you try
> and get your head round it - it took me a while and after having been in the
> "that's nuts" camp, it suddenly turned into one of those "ah, that makes
> sense" moments.
>
> In IPv4, who is a "neighbour" is intimately linked with subnets and subnet
> masks (prefix lengths). So if your address is 192.168.1.57/24 and my address
> is 192.168.1.123/24, then we are neighbours (assuming we are on the same
> physical network of course).
> Of course, you may also have the address 192.168.1.69/24 - so now we have
> duplicate information stored.
> And we are used to every IP address having an associated subnet mask.
>
> I don't know if you've ever worked with a multi-subnet network (where,
> usually because one got filled up, there are two IP subnets sharing one "bit
> of wire"), but if you have then you'll know that for hosts in the two subnets
> to communicate then they either have to be told explicitly about the "other"
> subnet(s), or they have to communicate via a router.
>
>
> With IPv6 things are different. IP addresses are distinctly separated from
> routing and the concept of who is a neighbour. An IP address does not have a
> prefix length, it is just an address. But, because of the way the network
> code (at least in the Linux world) has been built with this intimate linking
> of IP address and subnet mask - the IPv6 code "needs" a prefix length to be
> specified so that the value can be filled in in the internal data structures.
> In principle, whatever value is supplied, /128 should be used internally.
>
> You're probably, like I was, thinking this is nuts - so how does a host
> determine what else is on the network ?
> This is where routing advertisements come into play. A router will advertise
> to nodes a number of key information. There are some flags (such as whether
> nodes should auto-configure or look to something else such as a DHCP server
> for addressing), but one key thing is a list of prefixes used on the local
> network that should be considered local.
>
> So, taking your example address above, the host has obtained the address
> fda0:65da:e55a:1::20 (ignore the /128, that's just there because of this
> historical baggage in the code). That tells it nothing about what hosts it
> can talk to.
> The router should have sent out an advertisement stating that the prefix
> fda0:65da:e55a:1::/64 is in use AND that hosts can use neighbour discovery to
> find others in this subnet. It should be immediately obvious at this point
> that the router could advertise more than one prefix, and then hosts could
> automatically find and talk directly to neighbours in the other prefix(es).
>
> Why this emphasis on whether neighbour discovery can be used by the host ?
> For those brought up with broadcast networks like ethernet then it's probably
> a good question. There are some network types which from an addressing POV
> look the same, but don't support "broadcast" between different clients - AIUI
> many radio systems are like this. So if the host fda0:65da:e55a:1::20 wants
> to talk to fda0:65da:e55a:1::21 it cannot do so directly. So even though
> fda0:65da:e55a:1::20 and fda0:65da:e55a:1::21 are in the same prefix, they
> cannot directly talk to each other - instead they must send the packets up to
> the router.
>
> So while in the IPv4 world we (without configuration fudges) have only two
> combinations of same|different subnet and neighbours|not-neighbours. In the
> IPv6 world we have all 4 combinations possible - as well as the two obvious
> combination we can have hosts that are in the same prefix but are not
> neighbours, and hosts that are in different prefixes which are neighbours.
>
> Hope that helps a bit !
>
> _______________________________________________
> dhcp-users mailing list
> [email protected]
> https://lists.isc.org/mailman/listinfo/dhcp-users
--
Best regards
Sten Carlsen
No improvements come from shouting:
"MALE BOVINE MANURE!!!"
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