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Book excerpt from IPv6 for Enterprise Networks

Chapter 3: Common IPv6 Coexistence Mechanisms

By Shannon McFarland, Muninder Sambi, Nikhil Sharma, and Sanjay Hooda, Network World
May 05, 2011 04:27 PM ET

This chapter covers the following objectives:

  • Native IPv6: This section is for the greenfield deployment and covers the native IPv6 deployment.

  • Transition mechanisms: This section covers dual-stack (IPv4/IPv6 coexistence), IPv6 over IPv4 tunnels, and IPv6 over MPLS.

  • Protocol translation/proxy mechanisms: This section describes the translation mechanisms generally referred to by the layer at which they operate, for example, from the network layer to the application layer, and focuses on NAT-PT (Network Address Translation - Port Address Translation) and NAT64.

The current IPv4-based addressing scheme will not be enough to accommodate the growing addressing demands of the enterprise networks. IPv6 has been designed to address this shortfall. However, the challenge is how to add IPv6 to an enterprise network effectively and with the least impact. Various mechanisms have been proposed to achieve this. This chapter reviews the common coexistence mechanisms at a high level and includes the following:

  • Native IPv6 (IPv6-only networks): Native IPv6 refers to the network where IPv6 is the only transport protocol running.

  • Dual-stack: Dual-stack refers to the host/network where both IPv4 and IPv6 protocols are running on the devices.

  • IPv6 over IPv4 tunnels: In this transition mechanism, the IPv6 packets are encapsulated within an IPv4 packet. This method is used where IPv6 networks are segregated and IPv4 is the only option to transverse over the existing networks. In this scenario, the IPv6 is at the edge networks only.

  • IPv6 over MPLS: In this transition mechanism, the IPv6 domains communicate with peer IPv6 domains over an IPv4 Multiprotocol Label Switching (MPLS) core, providing more dynamic and higher performance.

  • Translation mechanisms: These mechanisms allow the communication of the IPv6-only devices with the IPv4-only devices by using translation mechanisms like Socks gateway, Network Address Translation - Port Translation (NAT-PT), TCP-UDP Relay, and NAT64.

Table 3-1 outlines and describes each of these on high level.

Table 3-1  Coexistence Mechanisms





Native IPv6

Scalability: IPv6 provides more addresses, thus providing global connectivity and peer-to-peer networking. Improved routing by aggregating routing entries (requires planning and design).

Might require network infrastructure upgrades, OS upgrades, application upgrades, services (DNS, DHCP), network management, and hardware upgrades such as NIC cards.

Transition mechanisms


Enables connectivity to existing IPv4 applications over IPv4 while providing access to IPv6-enabled applications over IPv6.

Supporting IPv6 and IPv4 simultaneously can be costly because it requires operation and management to support both protocols.

IPv6-over-IPv4 tunnels

Enables the IPv6 networks to connect with other IPv6 networks over IPv4 back- bone.

Large deployments, especially mesh configurations, can be complex and costly.

IPv6 over MPLS

Enables the IPv6 traffic to be encapsulated and traverse an IP/MPLS network without upgrading all of the MPLS core.

Might require an upgrade in software and hardware in the areas where the IPv6 network meets the MPLS network (for example, PE routers).

Translation mechanisms


Provides basic and limited translation between IPv4-only hosts and IPv6-only hosts.

Limited scalability and application support. Some of the other issues include DNS translation, and dual-stack hosts get both native and translated addresses. Limited performance dictated by NAT-PT device.


Provides two forms of translation modes between IPv6-only hosts and IPv4-only hosts: stateless and stateful modes. Stateless is a 1:1 mapping between IPv6 and IPv4. Stateful allows over- loading of multiple IPv6 addresses to one IPv4 address.

Some of the same limitations as any NAT device including NAT-PT, such as some application interoperability issues, scale, and performance. However, NAT64 is much better than NAT-PT.

Native IPv6

Native IPv6 is also known as "IPv6-only." It simply means that IPv6 is the only running IP protocol in the network.

In simplistic terms, the IPv6 environments can be thought of in the same way as native IPv4 environments of today, only using a higher version of IP. There will be a time when the operational and capital cost for running a dual-stack network will become unjustifiable. At some point, and that point varies greatly from network to network, it will make sense to turn off IPv4 altogether, or at least in the majority of the network.

There are already customers who deploy native IPv6 networks in "greenfield" or brand-new sites where the network and applications are the latest and greatest and also support IPv6 with few to no gaps in feature richness. These networks are a minority, but they will grow over time.

IPv6 enables organizations to deploy applications addressing specific business needs; however, the biggest challenge facing the deployment of native IPv6 in the near term is that not everyone is taking part in it. Additional challenges include the lack of end-to-end and robust IPv6 support in network, security, management, transport, and applications. Many of the large businesses have built proprietary applications, adding to the operational challenges for the transition to IPv6. This situation is applicable industry-wide, where there are gaps in the overall support for IPv6-only networks. Native IPv6 deployments will grow as support gaps are filled by vendors and service providers.

Figure 3-1 shows the native IPv6 model. The hosts and network devices in this model need to operate in IPv6 end to end for communication between them to be possible.

Figure 3-1
Native IPv6 Topology

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