16.2 Networking Protocols
16.2.2 The TCP/IP protocol
TCP/IP is not a single protocol but a suite comprised of a number of protocols operating at most of the layers of the OSI model. The Defense Advance Research Projects Agency (DARPA) originally developed TCP/IP to interconnect defense department computer networks across the country to form a fault tolerant network. It was adopted and further developed as the standard networking communications component of the UNIX operating system.

Many networking protocols have come and gone over the years. Proprietary protocols are rapidly losing ground in favor of open protocols like TCP/IP. Proprietary protocols include Novell's' IPX and Apple computer's AppleTalk. In fact, current versions of both Novell's and Apple's network operating systems support TCP/IP as a primary protocol. TCP/IP is an open standard and is the only protocol that is supported by every major operating system and nearly every networking equipment vendor. Virtually every computer manufactured in the world today, from laptops to the largest mainframes can communicate using the TCP/IP protocol. TCP/IP was originally developed as a WAN protocol but has recently become a required LAN protocol since most PCs on LANs need access to the Internet. With some LANs, TCP/IP runs along with other protocols such as IPX and AppleTalk, however it is becoming increasingly common to build LANs using only the TCP/IP protocol.

TCP/IP is the language of the Internet, which is the largest international network in the world with millions of nodes. The Internet is a packet-switched network that uses the Internet Protocol or IP to identify and encapsulate its packets. TCP/IP is the de facto UNIX standard and is the most widely used protocol suite in the world. Nearly every network equipment manufacturer has developed or is developing products that support TCP/IP.

The name TCP/IP is a combination of the two most important protocols in the suite: Transmission Control Protocol (TCP) and Internet Protocol (IP). TCP is one of the layer four transport protocols in the suite and IP is the layer three routing protocol in the Suite. TCP provides connection-oriented services to ensure that all packets reach their destination. Most Internet communication such as Hypertext Transfer Protocol (HTTP) use TCP as their transport mechanism. UDP is the other layer 4 protocol that provides connectionless services which can be used when delivery does not need to be guaranteed and for real-time applications such as voice and video transmission. Whether TCP or UDP, all packets ride over the Internet on IP. All computers (hosts) such as workstations and servers that wish to communicate over the Internet must have an IP address and must run the TCP/IP protocol. As a result, TCP/IP is fast becoming the protocol of choice for LANs and WANs.

TCP/IP is a “routed” or “routable” protocol. A host IP address has in it both the network’s address the address of the host on that network. This provides a way for one host to send packets to a host on another network while allowing each packet to be directed from the source host to a destination host sometimes through many routers along the way.

Any computer running the TCP/IP protocol is considered to be a “host”. A host is uniquely identified on the Internet by a 32-bit IP address. This address is divided into 4 set of 8 bits each which are referred to as octets. In addition to an IP address, TCP/IP computers are also given a host name such as www.sun.com. Most TCP/IP networks will allow a user to refer to the user-friendly host name as when browsing the Internet and a network naming service such as Domain Name System (DNS) or Network Information Server (NIS) will translate the host name into an IP address. Figure shows an example of an IP host address and a host name. Figure   shows two hosts communicating over the Internet using TCP/IP. Figures and compare the OSI model to the TCP/IP protocol stack.