[Magic Transit] IPv6 patch (#24344)

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Author: marciocloudflare

src/content/docs/magic-transit/how-to/ipv6.mdx

@@ -34,13 +34,33 @@ Cloudflare transports IPv6 traffic over an IPv6-over-IPv4 GRE tunnel. Here is ho
 
 Since IPv6 works over an existing IPv4 tunnel you will need to choose either an existing IPv4 GRE tunnel or create a new one to test IPv6. All settings that apply to the IPv4 GRE tunnel apply to the IPv6 tunnel as well, except for any MSS clamping you might need to configure — refer to [MSS clamping recommendations](#mss-clamping-recommendations) below for more information.
 
-To test and set up IPv6 in the Cloudflare dashboard there is only one new field you need to fill out when creating a new IPv4 GRE tunnel or editing an existing one: **IPv6 Interface address**. Here, you enter one of the two IPv6 addresses from the `/127` subnet that Cloudflare automatically allocates for the GRE tunnel. You need to enter the other IPv6 address in your router.
+To test and set up IPv6 in the Cloudflare dashboard there is only one new field you need to fill out when creating a new IPv4 GRE tunnel or editing an existing one: **IPv6 Interface address**. This is where you enter the Cloudflare-assigned IPv6 address for the Cloudflare side of the tunnel. Each tunnel is assigned a `/127` subnet from your allocated `/96` range. You will configure one address on the Cloudflare side and the other address on your router.
+
+:::caution
+Cloudflare allocates a `/96` IPv6 prefix for each account. The first two addresses in this range are reserved for Cloudflare. The remaining addresses are available for customer GRE tunnels, starting from `:2`, with two IPv6 addresses assigned per tunnel.
+:::
 
 To configure IPv6:
 
 1. Follow the instructions on how to [add a GRE tunnel](/magic-transit/how-to/configure-tunnel-endpoints/#add-tunnels).
-2. In **IPv6 Interface address** enter the IPv6 address assigned to you by Cloudflare for the Cloudflare side of the GRE tunnel. This address is part of a `/127` subnet that is provided to you by your Cloudflare team.
-3. Use the other IPv6 address you received from your team to configure your router.
+2. In **IPv6 Interface address**, enter the IPv6 address assigned to you for the Cloudflare side of the tunnel. This address is one of the two addresses in the `/127` subnet allocated from your `/96` allocation.
+3. Configure your router with the paired IPv6 address from the same `/127` subnet.
+
+### Example
+
+Your account has been assigned the prefix `2001:db8:abcd:1234::/96`.
+
+In this example, the first two addresses in the range (`::0` and `::1`) are reserved for Cloudflare. You can use any of the remaining addresses in the `/96` block to create `/127` subnets for your tunnels.
+
+If you decide to use the first available `/127` after the reserved addresses (`2001:db8:abcd:1234::2/127`), your configuration would be:
+- **Cloudflare IPv6 Interface address**: `2001:db8:abcd:1234::2`
+- **Router IPv6 address**: `2001:db8:abcd:1234::3`
+
+Continuing with the example, the next `/127` for the second tunnel would be `2001:db8:abcd:1234::4/127`. Thus, your configuration would be:
+- **Cloudflare IPv6 Interface address**: `2001:db8:abcd:1234::4`
+- **Router IPv6 address**: `2001:db8:abcd:1234::5`
+
+After the first two reserved addresses, you can continue allocating `/127` subnets sequentially (or in any order you prefer) for as many tunnels as needed until you reach the end of your `/96` range. Each `/127` contains exactly two IPv6 addresses — one for Cloudflare, one for your router.
 
 ### MSS clamping recommendations