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IPV6-USERS  July 2012

IPV6-USERS July 2012

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Subject:

Re: Subnet numbering. Is 'human readable' a good idea ?

From:

Phil Mayers <[log in to unmask]>

Reply-To:

Phil Mayers <[log in to unmask]>

Date:

Tue, 24 Jul 2012 16:11:28 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (62 lines)

On 24/07/12 15:33, Mac wrote:
> Dear IPv6-USers,
>
> (this is my first post here)
>
>
> Just wondering if the assembled wisdom on this list has any views about
> subnet numbering.

My "wisdom" is that it can distract you for a long time if you try to be 
too clever about it. I hear all kinds of wacky schemes proposed, and 
people get all kinds of excited about how "stingy" site /48 allocations 
are because they wanted to embed some crazy numbering scheme:

"""We need a /16 because I want to embed the latitude/longitude of the 
default gateway into the 6-byte subnet number"""

I exaggerate, but only just...

>
> I'm just at the very earliest stages of designing the IPv6 layout, and
> am trying to remember what I learnt on the 'test-run' of JANET/Tim
> Chown's IPv6 Fundamentals course some many months ago.
>
> We've got 2001:603:102::/48 assigned to us, and to start with I'm planning
> on carving it up in /64's more or less exactly inline with our existing
> IPv4 subnets.
>
> The numbering of the existing IPv4 subnets is well known and understood
> by the IT team here, so it makes sense to me to re-use it where
> possible.  The question in my mind however is:-
>
> 	For a subnet 217 (for example), do I use:-
>
> 	2001:630:102:217::/64
> 	or
> 	2001:630:102:d9::/64
>
> Whilst the former is more 'readable' it's not really 'correct'.
>
> 	Thoughts?

We got hung up on this for a long time, until I eventually decided it 
didn't matter.

Do whatever you prefer. Start at "1" and work your way up, if you want. 
Re-numbering is either trivial (boring desktop/laptop/mobile devices) or 
a nightmare regardless of address family (servers).

FWIW, we divided our /48 into /52s, and embed the vlan number in hex 
into the next 12 bits. The /52s are allocated for different uses 
"normal", "guest", "servers" etc.

normal vlan 123 becomes 2001:630:12:107b::/64
guest  vlan 456 becomes 2001:630:12:21c8::/64
etc.

This was purely a "tie breaker". The VLAN numbers are already unique 
within our network, so we just used it.

tl;dr - pick something simple and go with it. Don't overthink it ;o)

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