twork layer of the Internet is the IP - version 4 (IPv4) or version 6 (IPv6) is cur- rently present in every host, router, and gateway in every network. So, the Internet had won the packet-switched networking technologies battle by the end of the 1990s, which further resulted in the telecommunication/ICT world moving toward Internet-based networking and Internet technologies, a process that continued in the 2000s and 2010s. But was the Internet a separate network from the traditional telecommunication net- works? Well, corporations connected to the Internet via the Ethernet-based networks (where Ethernet is the IEEE 802.3 family of standards) or WiFi (which is the IEEE 802.11 1 Introduction Application layer Application layer Presentation layer (HTTP, FTP, email, DNS, DHCP, Skype, BitTorrent, etc.) Session layer Transport layer Transport layer (TCP, UDP) Network layer Network layer (IPv4, IPv6) Data link layer Link layer (Ethernet, WiFi, xDSL, FTTH, Physical layer 3G, 4G, cable, etc.) Traditional telecommunication Traditional Internet protocol protocol layering model (OSI) layering model 3G - Third-generation mobile systems HTTP - Hyper-text transfer protocol 4G - Fourth-generation mobile systems IPv4 - Internet protocol version 4 DHCP - Dynamic host configuration protocol IPv6 - Internet protocol version 6 DNS - Domain name system TCP - Transmission control protocol FTP - File transfer protocol UDP - User datagram protocol FTTH - Fiber-to-the-home xDSL - any digital subscriber line (e.g. ADSL) Figure 1.2 Comparison of traditional protocol layering model (Open Systems Interconnection (OSI) model) and Internet protocol layering model. family of standards) were initially denoted as computer networks (in the 1990s) because they were connecting computers (called hosts, either clients or servers). Meanwhile, the telecommunication networks typically connected "dumb" devices to the "smart" network, which included end-user devices such as telephones and TV sets. But res- idential users started to connect to the Internet 