frastructure. In addition, operators have additional infrastructure for their own services, and also infrastructure, for instance, for web acceleration. All these are also affected by the transition to IPv6. Sometimes the trickiest to transition are the billing and charging, operations and management, and the operators, own other infrastructure. The reason is that these infrastructures often include a lot of best practices proprietary to the operator - their own code that was written a long time ago, but no one knows by whom or when. If the software includes, for instance, literal IPv4 addresses or data structures only suited to IPv4, the code has to be modified, or even completely replaced. In this section, we will look at some of the parts of the system that may be affected by the transition to IPv6 and how to mitigate those impacts. It has been jokingly said that IPv6 transition is a year 2000 problem without a firm deadline. 5.5.2 IP Network Support Systems As stated previously, the 3GPP network needs a set of support systems that are vital for running an IP network. In a way, there are two completely disjoint IP network infras- tructures: the network that connects the network elements to each other, and the network elements to the management infrastructure which is its own network with its own sys- items. The second network is the network that provides the services for the UE. This network may include operator services, service enhancement functions, and the connec- tion to the Internet. These two networks are completely isolated from each other for security purposes. We will examine those networks separately below. Operator's Internal Infrastructure Network The operator's internal network may include one network or different network segments for operations and management for instance. This is the network that connects the different 3GPP elements to each other. Hence, it is the 3GPP transport plane that creates the IP layer in the protocol stack pictures in Chapter 2, and the lines between the elements in th