Subnetting Practices

[Kevin Loch]

Seth Mattinen wrote:
> I'm working on a subnetting scheme for my IPv6 deployment and I'm 
> curious to what the current best practices regarding IPv6 subnets are. 
> For example, if I need a point-to-point link, something I'd normally 
> assign a /30 in v4, I see /64 being used as the v6 equivalent. This 
> seems kind of wasteful to me, so if anyone out there can clarify why, 
> I'd appreciate it.

I use /112 for anything that will never use EUI-64.  I use /64 for any
subnet that might use it.  /112 is a convenient ':' boundary that makes
for easy visual identification and man management.  Unlike /126, It also
allows for expansion on multiple access type media (gig-E on vlans for
example).

There is no one right way to do it and It's important that we use
various prefix lengths other than /64 widely and often.  Sticking to /64
only will lead to really bad stuff being hard coded.

Speaking of whcih there is a limitation in 6500/7600 series devices that
applies to ACLs:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a00801609f6.html#wp1090842

So if you want to use port information in ACL's, you have to enable
address compression and avoid using bits 95-80 (not hard with /112-/126
prefix lengths).

- Kevin