## 1 Introduction  
In the last decade, security solutions and features in VPAN, WLAN, WMAN and distributed wireless systems have become increasingly mandatory [1, 2]. The number of Virtual Private Network (VPN) implementations has rapidly grown due to the enormous appeal of using \"public\" infrastructure to implement a secure link between the different locations of a company (e.g., a university) or between a \"road warrior\" and its seat [3], also involving cellular architectures. There are several advantages to implementing a VPN, such as the following:  
1. Cost effectiveness (cost of infrastructure is lower, and the appropriate choice of the implementation phase allows one to choose the best solution at the lowest sustainable cost);
2. Simplicity (the technology is very mature and does not require esoteric skills);
3. Safety (the technology is based on open standards and is mostly universally safe). Therefore, with little effort, it is possible to achieve a good compromise between ease of access and reasonable safety. The VPN concept is quite simple: it is based on the paradigm of \"hub and spoke\", with a central location from which a huge set of connections departs to remote locations. By using appropriate rules, it is possible to decide whether each remote site exclusively accesses the central node or whether traffic between the suburbs should be enabled. The solutions for implementing these kinds of accesses are very wide and different, ranging from a simple server equipped with open-source software to expensive redundant appliances in High Availability (HA). The precise choice depends on the costs, the grade of integration with existing infrastructure, the required bandwidth, the workload and/or the criticism of the link (to mention just a few factors that affect the choice of VPN system). Moreover, recently, the energy consumption of the adopted security solutions has become an aspect to consider [4]. In fact, with the general operation of a VPN, all traffic between the two endpoints of the VPN is encapsulated into pre-established tunnels that can be on different levels of the ISO/OSI model (IPSEC or IKEv2 IPSEC at layer three, PPTP at layer five, L2 TP at layer two and OpenVPN or Virtual Tunnel Daemon at layer four) [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. This paper is focused on giving a deep overview of the possible implementations of VPNs and Virtual Local Area Networks (VLANs) and their recent progresses in wireless and mobile scenarios. In particular, our main aim is to give a detailed review of the efficient implementation of VPN for a mobile scenario. In fact, conventional VPN solutions (as referenced above) are dedicated to static networks (with no mobile nodes). It is known that in a mobile scenario, connections are not as reliable as in the wired case: mobility effects, obviously, reflect on VPN performance, resulting in low-speed connections, packet loss and low throughput. With the rapid proliferation of mobile devices and high speeds (e.g., with the fifth generation (5G)), users are more oriented to use not just their home devices but also their mobile devices (such as smart phones and tablets), considering opportunistic communication and services [15, 16]. Cloud Computing (CC) has reached a high grade of development, and nowadays, users connect with their mobile devices to the cloud in order to access their cloud services. In the above scenario, it is clear that establishing a reliable session is essential, and VPNs are the first \"tool\" that can be used to guarantee security and robustness. Unfortunately, VPNs work well only with stable network connections. If connection losses or service degradations occur, VPN connections will surely break, and users will not be satisfied with the results, which can involve data loss and continuous session breaks. Therefore, the main aim of this work is to review and survey the main VPN solutions for static (stationary nodes) and dynamic (mobile nodes) networks from a practical point of view. A valid alternative to VPNs is VLAN technology.