In an increasingly interconnected world where the demand for IP addresses has far exceeded the limits of the outdated and obsolete IPv4 protocol, IPv6-mostly networks are emerging as a solution to meet the challenges of the much sought after transition to IPv6.
What Is an IPv6-Mostly Network?
Section 1.2 of RFC 8925 provides the following definition:
IPv6-mostly network: A network that provides NAT64 (possibly with DNS64) service as well as IPv4 connectivity and allows the coexistence of IPv6-only, dual-stack, and IPv4-only hosts on the same segment. Such a deployment scenario allows operators to incrementally turn off IPv4 on end hosts, while still providing IPv4 to devices that require IPv4 to operate. But IPv6-only-capable devices need not be assigned IPv4 addresses.
Let’s not forget that one of the main reasons for IPv6 adoption is the lack of available IPv4 addresses. IPv4 uses 32-bit addresses, which—in theory—allows a maximum of 4.3 billion unique IP addresses worldwide. The growth of Internet-connected devices such as smartphones, tablets, computers, and others has made it increasingly difficult to assign a unique IP address to each device.
On the other hand, IPv6 uses 128-bit addresses, which means it offers enough unique IP addresses to assign to each Internet-connected device. IPv6 allows for a maximum of 340 sextillion unique IP addresses, which is more than enough to handle the growing number of Internet-connected devices at least until the year 200,000 (and yes, we’re currently in 2025).
IPv6 also improves network efficiency by reducing overhead and improving network routing capabilities. This means that data packets can be transmitted and switched at greater speed and with fewer interruptions, improving the quality of the end-user experience.
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And How Does an IPv6-Mostly Network Operate?
To understand how an IPv6-mostly network operates, we need to review how DHCPv4 works.
DHCPv4 Process
This process includes four main steps, known as DORA, which stands for Discovery, Offer, Request, Acknowledge. Below is a brief summary of each of these steps.
And How Does an IPv6-Mostly Network Operate?
To understand how an IPv6-mostly network operates, we need to review how DHCPv4 works.
DHCPv4 Process
This process includes four main steps, known as DORA, which stands for Discovery, Offer, Request, Acknowledge. Below is a brief summary of each of these steps.
1. DHCP Discover
The client sends a DHCPDISCOVER packet as a broadcast (destination: 255.255.255.255, UDP port 67) to locate available DHCP servers on the network.
2. DHCP Offer
Each DHCP server on the network responds with a DHCPOFFER (broadcast or unicast, UDP port 68). It provides an available IP address, subnet mask, gateway, DNS, and lease time.
3. DHCP Request
The client sends a DHCPREQUEST as a broadcast (to notify all servers).
It specifies the selected server (by its IP) and the offered IP address. The purpose of this step is to confirm acceptance of the offer from step 2.
4. DHCP Acknowledgment
The selected server replies with a DHCPACK (acknowledgment) to confirm.
But Why Are We Talking about IPv4 and DHCPv4 if This Article Is about IPv6?
To understand how an IPv6-mostly network functions, we need to understand that it all begins in the world of IPv4. Through the DHCPv4 protocol, the client signals that it’s possible to participate in an IPv6 network even without any IPv4 addresses. This is accomplished using DHCPv4 options (RFC 2132). In the same way DHCPv4 delivers an IPv4 address, a subnet mask, and DNS servers, it can also say: “I can use IPv6-only if this is what the network wants me to use.”
What Happens in an IPv6-Mostly Network?
In an IPv6-mostly network, using option 108, the client tells the DHCPv4 server: “Hey, I can operate on an IPv6 network without IPv4.”
The server recognizes option 108, and this triggers a “short circuit” in the DHCPv4 DORA process. The client skips the IPv4 Request, and naturally, the server never sends an IPv4 Acknowledge. Voilà! One more IPv4 address remains available in the DHCP server’s pool.
Example of Option 108 Configuration on a DHCPv4 Server
It’s worth noting that the option is relatively new on the DHCPv4 client, and some operating systems still do not support it. Currently, it is estimated that approximately 75% of devices support this option.
Advantages of an IPv6-Mostly Network
I won’t go into detail about the number of advantages, as there are many and I’m sure the reader will be able to infer many of them. That said, I will mention the top two:
Significant savings in IPv4 addresses, and
Reduced need for NAT thanks to decreased IPv4 utilization.
Conclusions
To summarize, an IPv6-mostly network is one where the IPv6 protocol is used predominantly instead of IPv4. By adopting IPv6, businesses and organizations can leverage the benefits of a larger address space, enhanced security, and greater network efficiency. As the number of devices connected to the Internet continues to grow, IPv6 is becoming an increasingly attractive option for businesses and organizations looking to build a secure and efficient network capable of handling the growing volume of Internet traffic.
