IPv6: Transitioning from IPv4 to IPv6

The theory is that when you begin to migrate your IPv4 network towards IPv6, you start at the edge of the network and work your way towards the core. This requires thought on how we will have IPv4 and IPv6 working with each other side by side.

To achieve this, we have 2 approaches with 3 overall methods.

  1. Translating
  2. Encapsulating
  • Dual Stack
  • 6-to-4 Tunnel
  • NAT-PT

Dual Stack

A host runs dual stack when it runs both IPv4 and IPv6. Dual stack helps meet the migration challenge we face when end users want to keep using their favorite IPv4-based apps while the network moves forward to IPv6-based apps.

6-to-4 Tunnel

Cisco documentation states that setting up a 6-to-4 tunnel is very simple on the host ends of the tunnel. A 6-to-4 tunnel is also automatic, is torn down when the session ends and is a scalable solution.

6-to-4 tunnelling is accomplished by taking an IPv6 packet and encapsulating it into an IPv4 packet (protocol type 41) for transport across the IPv4 section of the network, then de-encapsulating it when the remote edge router is ready to route it across the IPv6 network. The IPv6 networks shown in this method are sometimes referred to as IPv6 islands.

6-to-4 tunnels also have a reserved IPv6 address prefix for edge routers. These prefixes begin with 2002 and are followed by the router’s IPv4 address expressed in hex. These prefixes carry a /48 prefix, such as 2002:1234:83cd::/48.

Hex Example of IPv4 to Hex Conversion

The IPv4 address of the interface involved in the tunneling is vital in determining the correct IPv6 address for the tunnel. Let’s say the IPv4 address of one of the routers is 220.200.18.42. We know the address for the corresponding tunnel interface begins with 2002, to complete the rest we break down each octet into hex:

  • 220 = 13 units of 16, 12 units of 1 = hex value is DC
  • 200 = 12 units of 16, 8 units of 1 = hex value is C8
  • 18 = 1 unit of 16, 2 units of 1 = hex value is 12
  • 42 = 2 units of 16, 10 units of 1 = hex value is 2A

The IPv6 address for the tunnel interface is 2002:DCC8:122A::/48.

NAT-PT

NAT-Protocol Translation works much the same as normal NAT.  If you have IPv6 hosts that need to intercommunicate with IPv4 hosts on another segment, NAT-PT may be the perfect solution.

NAT routers translate private IPv4 addresses to public IPv4 addresses, and back again; NAT-PT routers translate IPv6 addresses to IPv4 addresses, and back again.

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