between them. Though this distinction is relatively straightforward, nodes can sometimes have both roles. A good example is the home routers that connect a home network to a wide area network. These usually function as routers, but also often implement, for instance, a web server for management purposes. As a web server the device is a host, but as a home gateway it is a router. It is important to note that IP does not have any special distinction between clients and servers. From IP's point of view, they are all hosts. In addition to routers, hosts can also have multiple network interfaces connected to multiple IP networks at the same time. Hosts with multiple interfaces, and multiple IP addresses, are called multi-interfaced hosts. Logically, a host with just one interface called a single-interfaced host or just a host. It is actually quite usual for hosts, such as personal computers, to be multi-interfaced. For instance, today it is normal for a laptop to be connected to a wireless and a wireline networks at the same time. In addition, when a host has a virtual interface, such as a VPN tunnel, it is always multi-interfaced: it has at least the one physical interface and the virtual interface. The IP nodes are connected to each other by a network of lower layer network con- nections called links. These links can be either point-to-point links connecting two nodes, or complete layer 2 networks shared between multiple nodes. Different link-layers are used in modern networks. By far the most commonly used link-layer technology Ethernet [6]. As Ethernet is a very common technology in IP, some parts of the IP technology expect Ethernet-like functionality as well as from other link-layer technolo- gies, and even some non-Ethernet technologies simulate Ethernet for easier integration to their operating systems' IP implementations (which is not without issues, as we will see in Section 4.9.1). 1.3.1 Networks of Networks When reading about IP, and the biggest IP network of all - the Internet - sometimes the usage of the