Thread by @bridgeisroot

Are you trying to develop an #IPv6 subnetting plan? Here are a couple of subnetting plans and best practices to help you plan your deployment.

Best practice is to subnet on nibble boundaries (one hex digit). It's way easier to differentiate the location and purpose of a subnet this way. It's difficult to remember that :[0-7]A: is for one purpose and :[8-9A-F]A: is for something else.

Minimum prefix size that can be advertised on the internet is /48. Your organization might not have internet breakout in your design today, but that could change. I recommend that you assign a minimum of a /48 per location.

A /48 carved out of PI space can be translated at the internet edge using Network Prefix Translation to PA space. Especially useful for SD-WAN deployments and consistent addressing across the enterprise.

Your move to IPv6 is a great chance to migrate to an L3 to the closet network architecture. Design your subnetting plan to accommodate.

Point to point links should get a /64 assigned but I like to configure as a /127. It might be tempting to use IPs ::1 and ::2 but these are actually in different subnets. ::A and ::B are better.

As for subnetting plans, first is classic location based subnetting. Great for most organizations that don't have complex macro-segmentation needs.

2001:db8:0:[Location][Building][Floor or Closet][Function]::/64

This is the most compact and easy to spot by eye the function of the subnet based on the last character of the prefix. Summarization is easy based on closet.

Next up is a function-based plan that still maintains easy summarization based on the location. We move the function up to directly after the location bits.

2001:db8:0:[Location][Function][Building][Floor or Closet]::/64

Useful if you have VRF based macro-segmentation policies at your branch locations but maintains the ability to breakout to the internet at each location.

Lastly, we have a plan that is useful when you have a large campus environment with macro-segmentation policies. We move the function bits all the way to the front.

2001:db8:0:[Function][Location][Building][Floor or Closet]::/64

Useful if your architecture uses a centralized firewall for maintaining segmentation across multiple locations or buildings.

Dedicate as many or as few bits to each level as you need but try to keep them in 4 bit increments. More than 16 but fewer than 256 locations? Dedicate 2 nibbles to the location. Need up to 4k locations? 3 nibbles should do the trick.

Think through your organization growth for many years out. Every company has different needs and may have a natural ceiling the the size of the organization.