@book { VirtualPrivateNetworking,
    title = "Virtual Private Networking",
    author = "Markus {Igelsböck}",
    year = "2001",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832446574",
    url = "https://m.diplom.de/document/220266"
}