Behind the Scenes of IPv6 Synthesization: From Prefixes to Addresses

As the world steadily transitions to an IPv6-based internet, one of the critical processes ensuring smooth communication between IPv6-only clients and legacy IPv4 servers is IPv6 address synthesization. This process allows devices using IPv6 to connect to IPv4 systems without disruption, thanks to technologies like NAT64 and DNS64. But how does this work, exactly? In this blog, we’ll explore IPv6 synthesization in-depth — from the determination of prefixes to the construction of IPv6 addresses.

The Role of IPv6 Synthesization in Transition Mechanisms

IPv6 synthesization is essential for networks where IPv6-only clients need to communicate with IPv4-only servers. It bridges the gap by creating IPv6 addresses that can be translated into IPv4 addresses, facilitating seamless communication across both protocols.

IPv6 synthesization mainly involves two key components:

1. NAT64 Prefix Determination
2. IPv6 Address Construction Using the Synthesized Prefix

Let’s take a closer look at both of these processes.

1. NAT64 Prefix Determination: Setting the Foundation

The first step in synthesizing an IPv6 address is determining the NAT64 prefix. A prefix is a portion of the IPv6 address used to route traffic, and in this context, it’s crucial for translating traffic between IPv6 and IPv4 networks. This prefix is referred to as Pref64::/n, where /n refers to the length of the prefix in bits.

There are two types of prefixes used in IPv6 synthesization:

• Network-Specific Prefix (NSP): This is a unique prefix assigned by an organization. It’s typically a subnet from the organization’s existing IPv6 allocation.
• Well-Known Prefix (WKP): The well-known prefix used for NAT64 is 64:ff9b::/96. It’s a standard prefix defined by RFC 6052, reserved for NAT64 traffic.

Understanding Pref64::/n

For IPv6 synthesis to happen, nodes (devices in the network) must first learn the NAT64 prefix(es) used on the network. For example, the prefix may be a WKP (like 64:ff9b::/96) or an NSP specific to the organization deploying NAT64.

Nodes can also determine the NAT64 prefix by resolving the domain name ipv4only.arpa. This domain resolves to two special IPv4 addresses, 192.0.0.170 and 192.0.0.171. If a node uses a DNS64 server, the server will return synthesized IPv6 addresses containing the NAT64 prefix (such as Pref64::192.0.0.[170|171]). The node extracts the prefix from this response to synthesize its own IPv6 addresses.

Handling Multiple Prefixes

In cases where the DNS64 server returns multiple prefixes, the node uses all of them when determining if an address is synthetic. This allows the node to synthesize multiple addresses using all the prefixes, following the order in which they were received.

2. Synthesizing the IPv6 Address: Putting It All Together

Once the prefix has been determined, the next step is constructing the IPv6 address. A synthesized IPv6 address consists of three key parts:

• The NAT64 prefix (either WKP or NSP)
• The IPv4 address, embedded in the IPv6 address
• A suffix, which is often set to zero, depending on the prefix length

Here’s how the address is structured:

<IPv6-Prefix>:<IPv4-Address>:<Suffix>

The prefix could be 64:ff9b::/96 for a well-known prefix, or it could be a unique network-specific prefix. The embedded IPv4 address follows, and the suffix depends on the length of the prefix. Let’s break down the different scenarios based on the prefix length:

Example of Prefix Lengths and IP Address Embedding:

1. Prefix Length /32:

• The IPv4 address is placed in positions 32 to 63 of the IPv6 address.

1. Prefix Length /40:

• The first 24 bits of the IPv4 address are encoded between positions 40 to 63, and the remaining 8 bits are placed between positions 72 to 79.

1. Prefix Length /48:

• The first 16 bits of the IPv4 address go between positions 48 to 63, with the remaining 16 bits in positions 72 to 87.

1. Prefix Length /56:

• The first 8 bits of the IPv4 address are in positions 56 to 63, and the remaining 24 bits are encoded between positions 72 to 95.

1. Prefix Length /64:

• The entire IPv4 address is encoded between positions 72 and 103.

1. Prefix Length /96:

• The IPv4 address is embedded between positions 96 and 127.

Each of these formats ensures that an IPv4 address is correctly mapped into an IPv6 address based on the prefix used, allowing traffic to be routed through NAT64 gateways.

Address Translation: Algorithms at Work

The synthesis of an IPv6 address involves more than just embedding an IPv4 address in a longer format. A specific algorithm is used to ensure that the IPv4 address fits seamlessly within the structure of the IPv6 address. Here’s how it works:

1. Concatenate the Prefix and IPv4 Address: Combine the NAT64 prefix with the 32-bit IPv4 address.
2. Adjust for Prefix Length: If the prefix is less than 96 bits, a “null octet” (designated as u) is inserted between bits 64 and 71 to ensure compatibility with the host identifier format.

On the flip side, when extracting the IPv4 address from a synthesized IPv6 address, the reverse algorithm is applied. For example, if the prefix is 96 bits long, you can simply extract the last 32 bits to get the original IPv4 address.

Conclusion: Synthesization as the Backbone of IPv6-IPv4 Interoperability

IPv6 synthesization plays a pivotal role in allowing IPv6-only clients to communicate with IPv4-only servers, and vice versa. By dynamically generating IPv6 addresses from IPv4 counterparts using NAT64 prefixes, DNS64, and intelligent algorithms, the internet can continue to function smoothly during this ongoing transition phase from IPv4 to IPv6.

Understanding the nuances of IPv6 synthesization is crucial for network administrators and engineers involved in managing the transition to IPv6. As IPv6 adoption continues to rise, these technologies will remain essential in ensuring that both IPv4 and IPv6 systems can coexist and communicate efficiently.