sition Checklist As stated earlier, every network operator, and thus every network, is different. There are multiple variables that create the unique characteristics of an operator. For instance, IPv6 Transition Mechanisms for 3GPP Networks geography, population, and demography of the country could have influenced the operator's network greatly. In addition, the time when the network has been taken into use, and how it has been set up, influence the state of the current network. Therefore, it is difficult to give a planning template for IPv6 transition that would fit all or even most operators. However, there are certain steps that are common for all operators regardless of the characteristics of the network. Here we try to provide a checklist of steps that an operator should consider when planning and executing the transition to IPv6. Prioritize end-user service over operator internal infrastructure. The biggest pressure on addressing is on the end-user services, as the UEs use the most addresses. As the oper- ator's internal network is more static, and there are fewer nodes in the network, the use of addresses is much less, and the address planning even with few addresses is easier. Survey the network carefully. It is especially important to check all the support functions such as billing and charging, operations and management, and subscriber management functions, as these are the systems that are most likely to have proprietary software, created over the years. The proprietary software is also the most difficult to upgrade to support IPv6, as company-internal software tends to be less well documented than com- mercial software. In addition, maintaining software needs fewer people than upgrading the software. It is important to understand not only which systems will be directly connected to the IPv6 network, but also which systems will have to deal with IPv6 addresses. For instance, the operations and management system may be connected to the network elements with IPv4, but it has to be able to configure an el