28. TCP/IP
The original design of TCP/IP as a Network of Networks fits nicely within the current technological uncertainty. TCP/IP data can be sent across a LAN, or it can be carried within an internal corporate SNA network, or it can piggyback on the cable TV service. Furthermore, machines connected to any of these networks can communicate to any other network through gateways supplied by the network vendor.
The Internet Protocol was developed to create a Network of Networks (the "Internet"). Individual machines are first connected to a LAN (Ethernet or Token Ring). TCP/IP shares the LAN with other uses (a Novell file server, Windows for Workgroups peer systems). One device provides the TCP/IP connection between the LAN and the rest of the world.
TCP. The Transmission Control Protocol is a reliable, connection-oriented delivery service. It uses byte-stream communications so data is treated as a sequence of bytes. For each data segment sent, the receiving host must return an acknowledgment within a specified period. If there is no acknowledgment, the data is retransmitted. A TCP session is initialized via a three-way handshake in order to synchronize the sending and receiving of data segments. All TCP data segments have two parts: data and header.

Sockets applications use a unique port number. Port numbers for well-known server side applications are pre-assigned by IANA and do not change. Port numbers for client-side applications are dynamically assigned by the operating system. A socket is created by an application by specifying the IP of the host, the service type (TCP or UDP) and the port the applications is using.