# Enhancing Data Security Through VLSM Subnetting and TCP/IP Model in an ENT
## 2 Literature Review  
### Transmission Control Protocol/Internet Protocol  
The updated TCP/IP supports communications amongst connected devices across networks to the internet. The TCP/IP model has five layers [23]: the physical layer, data link layer, network layer, transport layer, and application layer (see Figure 1). The layers in this model are responsible for maintaining communication channels, flow control, and reliability assessment via protocols.  
Information systems using the TCP/IP protocols are categorised into a host and an intermediate node. The intermediate nodes involve the first three layers of the TCP/IP model. A service interface is present in-between each layer, which is varied by an operating system. Services delivered to the top layer initiate commands, data, and key parameters to the bottom layer to perform the required task. The layers on the same system can attain and offer services from and to adjacent layers based on a peer-to-peer protocol process enabling the same layers on different network systems to communicate. While a service interconnects layers within the same process, a protocol links processes.  
Figure 1 includes a Domain Name Service (DNS) for the conversion of a domain name to an IP address and the Hyper-Text Transfer Protocol (HTTP/HTTIPS) used by Web browsers to view information; HTIPS indicates securely sending data to a domain (website) from a web browser by encrypting and safely delivering data; Dynamic Host Configuration Protocol (DHCP) ensures network configuration is performed in a centralised domain to reduce the risk of misconfiguration, whereby the client host on this ENT can obtain an IP address dynamically from the server. Other protocols associated with the Application Layer include Secure Shell (SSH) and Teletype Network (Telnet). SSH relies on the Internet, which leans on the TCP/IP protocol thereby providing a secure connection for communication to a host over an unsecured network. Telnet enables a host within a network to connect to another local host within the same network, while SSH has capabilities to access various services in the cloud without unveiling the ports of local hosts to the Internet. It can remotely communicate with services in a private network and evade the restrictions of firewalls. Ports 22 and 23 are assigned to SSH protocol and Telnet protocol, respectively. SSH enables cryptographic authentication and encryption connections between hosts and tunnelling where data packets are sent across networks. SSH transmits data in an encrypted format through a secure channel, whereas Telnet transfers the data in plain text.  
A Point-to-Point protocol (PPP) connects routers directly, thereby providing data compression, data-in-transit encryption, and authentication detection. The Serial Line Internet Protocol (SLIP) encapsulates the Internet Protocol relating to serial ports and router connections. The PPP does not require configuration of an IP address to establish a link, while SLIP encapsulates IP packets with a reduced overhead--the performance of each node in the network such as time, memory, energy, and bandwidth. The physical layer uses a Wi-Fi IEEE 802.11 and Bluetooth protocols enabling connection and communication with wireless network interfaces [23].  
Figure 1: The updated TCP/IP model layers with protocols.