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+------------------------------------------------------+ ▌ Oblivion Underground Magazine - Issue 1 - 15/03/2000 ▌ ▌ Introduction to TCP/IP by Cyber0ptix ▌ ▌ E-Mail : cyberoptix@email.com ▌ +------------------------------------------------------+ Introduction to TCP/IP! ----------------------- The Transmission Control Protocol/Internet Protocol is, in a nutshell, the means by which information is passed from one machine to another (platform independent) accross the internet. TCP is a communications protocol that reliable transfer of data (In reality not that reliable). It is responsible for assembling data passed from higher layer applications into standard packets and ensuring that the data is tranfered correctly. IP is the Internet Protocol which is responsible for moving the packets of data assembled by TCP accross networks. It uses a set of unique addresses for every device on the network to determine routing and destinations. IP Addresses ------------ An IP (Internet Protocol) Address is used to identify a particular machine on a network and the network to which it is attached. (IP addresses identify a machine's connection to the network, not the machine itself!). So whenever a machine's location on the network changes, the IP address must be changed too. The IP address is the set of numbers which uniquely identifies that machine. IP addresses are assigned only by the Network Information Center (NIC) although if a network is not connected to the internet, that network can determine its own numbering. For all internet accesses, the IP address must be registered with the NIC. There are four formats for the IP address, with each being used depending on the size of the network. The four formats, Class A, B, C and D. Class A addresses are for larger networks that have many machines. Class B are for intermediate networks. Class C are limited to 256 devices and class D are used for multicasting (Sending a general broadcast to more than one machine). IP addresses are four sets of 8 bits, giving a total of 32 bits. These bits are often represented by a . dot for convenience, so the IP address format can be thought of as: network.local.local.local for Class A or network.netowrk.network.local for Class C The IP addresses are usually written out in their decimal equivalents, such as 195.166.128.53. This particular example would indicate that the network address is 195.166 and the local or host address is 128.53. More about TCP/IP ----------------- TCP/IP is the basic communication language or protocol of the internet. It can also be used as a communications protocol in the private networks refered to as Intranets. When you are set up with direct access to the internet, your computer is provided with a copy of the TCP/IP protocol suite, just as every computer that you may send messages to or get information from also has a copy of TCP/IP. TCP/IP is a two layered program. The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets that are transmitted over the internet and recieved by a TCP layer that reassembles the packets into the original message. TCP is a method used along with Internet Protocol to send data in the form of message units between computers over the internet. While IP takes care of handling the actual delivery of data, TCP takes care of keeping track of the individual units of data (called packets or datagrams) that a message is divided into for efficient routing throught the internet. For Example, when an HTML file is sent to your from a Web Server, the TCP program layer in that server divides the file into one or more packets, numbers the packets, and then forwards them individually to the IP program layer. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end (the client program on your computer), TCP reassembles the individual packets and waits until they have arrived to forward them to you as a single file. TCP is known as a connection-oriented protocol, which means that a connection is established and maintained until such time as the messsage or messages to be exchanged by the application programs at each end have been exchanged. TCP is responsible for ensuring that a message is divided into packets that IP manages and for reassembling the packets back into the complete message at the other end. In the Open System Interconnection (OSI) communications model, TCP is in layer 4, the Transport Layer. The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the right destination. each of these packets contains both the senders address and the recievers. Any packet is sent first to a gateway computer that understands a small part of the internet. The gateway computer reads the destination address and forwards the packet to an adjacent gatewway that in turn reads the address and so forth across the internet until one gateway recognises the packet as belonging to a computer within its immediate neighbourhood or domain. That computer then forwards the packet directly to the computer that is specified in the address. Because a message is divided into a number of packets, each packet can, if necessary, be sent by a different route accross the internet. Packets can arrive in any order. The Internet Protocol just delivers then. Its upto the Transmission Control Protocol to put them back in the right order. IP is a connectionless protocol, which means that there is no established connection between the end points that are communicating. Each packet that travels through the internet is treated as an independant unit of data. (The reason the packets get put back in the right order is because of Transmission Control Protocol, the connection-oriented protocol that keeps track of the packet sequnce in a message). In the Open Systems Interconnection (OSI) communications model, IP is in Layer 3, the Network Layer. TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided with a service (such a sending a Web Page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher level applications that use it are collectively said to be "connectionless" because each client request is considerd to be a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the duration of the call). Being conectionless frees network paths so that everyone can use them continously. (Note that the TCP layer itself is not connectionless as far as any one message is concerned. Its connection remains in place until all packets in a message have been recieved.) Thats everything for this months introduction to TCP/IP. Get next months issue for a detailed description of IP address and Subnets. Also DNS explained...so get ready to download Issue 2 of Oblivion Underground Magazine. Cyber0ptix paths so