When you get IPv6 connectivity from your ISP you should get a /48 prefix of IPv6 addresses per site by default. When you have multiple sites you should get a /48 for each. For many organisations this is enough: it provides you with 65,536 subnets per site. You are probably not going to use all of them, but having such a large block allows you to make a good addressing plans where you can group addresses in a logical and useful way, not limited by scarcity.

Getting a block of IPv6 addresses from your ISP is easy. ISPs get a Provider Aggregatable (PA) block from their Regional Internet Registry, and this block provides plenty addresses to give each customer a /48. They usually provide that /48 as part of the standard service.

The downside of using your ISP’s PA addresses is that you use their addresses. When you want to connect to multiple ISPs for redundancy you can’t get real redundancy because each ISP will provide you with addresses from their own block. The addresses from one ISP are not accessible through the other ISP.

This also happens when you want to change ISPs: you’ll have to give the old block back to the ISP you are leaving and you’ll get a new block from your new ISP. You’ll have to renumber from the old addresses to the new ones. Because NAT isn’t used with IPv6 this includes both your externally accessible services and your internal networks.

With IPv6 renumbering is easier to do than with IPv4. IPv6 is designed so that using multiple addresses is normal. You can therefore connect the new ISP, add their addresses to your network, and after everything has been migrated you can remove the old ISP’s addresses and cancel your connection to them. Nevertheless, this can be a lot of work.

Summary

  • Using your ISP’s PA addresses is a good idea when:
    • You only connect to one ISP
    • You don’t change ISPs often
    • You don’t want the responsibility of getting the addresses routed on the internet
  • Using your ISP’s PA addresses might not be a good idea when:
    • You want to connect to multiple ISPs
    • You change ISPs regularly