TCP/IP model:

Layers
The TCP/IP model defines four totally independent layers into which it divides the process of communication between two devices. The layers through which it passes information between two devices are:

Application
The application layer is the layer nearest the end user. This is the layer that is in charge of translating data from applications into information that can be sent through the network. The basic functions of this layer are:

• Representation
• Codification
• Dialogue Control
• Application Management

Transport
The transport layer establishes, maintains and finishes virtual circuits for information transfer. It provides control mechanisms for data flow and allows broadcasting, and it provides mechanisms for the detection and correction of errors. The information that arrives at this layer from the application layer is divided into different segments. Information that comes to the transport layer from the internet layer is delivered back to the application layer through ports. The basic functions of this layer are:

• Reliability
• Flow Control
• Error Correction
• Broadcasting

Internet
This layer divides the segments of the transport layer into packets and sends the packets across the networks that make up the Internet. It uses IP, or internet protocol addresses to determine the location of the recipient device. It does not ensure reliability in the connections, because this is already taken care of by the transport layer, but it is responsible for selecting the best route between the originating device and the recipient device.

Network Access
This layer is in charge of sending information at both the LAN level and the physical level. It transforms all the information that arrives from the superior layers into basic information (bits) and directs it to the proper location. At this level, the destination of the information is determined by the MAC, or media access control, address of the recipient device.

Protocols
To be able to send information between two devices, both must speak the same language. This language is called the protocol.
The protocols that appear in the application layer of the TCP/IP model are:

• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (smtp)
• Domain Name Service (DNS)
• Trivial File Transfer Protocol (TFTP)

The protocols of the transport layer are:

• Transport Control Protocol (TCP)
• User Datagram Protocol (UDP)

The protocols of the internet layer are:

• Internet Protocol (IP)

The protocol most often used in the network access layer is:

• Ethernet

Note: The protocols listed above and their associated ports will be described in the following sections.

Application layer protocols: 

FTP or file transfer protocol is used for the transmission of files between two devices. It uses TCP to create a virtual connection for the control of information, then creates another connection to be used for the delivery of data. The most commonly used ports are 20 and 21.

HTTP or hypertext transfer protocol is used to translate information into web pages. This information is distributed in a manner similar to that used for electronic mail. The most commonly used port is 80.

SMTP or simple mail transfer protocol is a mail service that is based on the FTP model. It transfers electronic mail between two systems and provides notifications of incoming mail. The most commonly used port is 25.

DNS or domain name service provides a means to associate a domain name with an ip address. The most commonly used port is 53.

TFTP or trivial file transfer protocol has the same functions as FTP but uses UDP instead of TCP. This gives it more speed, but less security and trustworthiness. The most commonly used port is 69.

Transport layer Protocols:

There are two protocols which can be used by the transport layer to deliver information segments:

TCP or transmission control protocol establishes a logical connection between the final points of the network. It synchronizes and regulates the traffic with what is known as the “Three Way Handshake”. In the “Three Way Handshake,” the originating device sends an initial packet called a SYN to the recipient device. The recipient device sends an acknowledgment packet, called a SYN/ACK. The originating device then sends a packet called an ACK, which is an acknowledgment of the acknowledgment. At this point, both the originating device and the recipient device have established that there is a connection between the two and both are ready to send and receive data to and from each other.

UDP or user datagram protocol is a transport protocol which is not based on a connection. In this case, the originating device sends packets without warning the recipient device to expect these packets. It is then up to the recipient device to determine whether or not those packets will be accepted. As a result, UDP is faster that TCP, but it cannot guarantee that a packet will be accepted.

Internet layer Protocols:

IP or internet protocol serves as a universal protocol to allow any two computers to communicate through any network at any time. Like UDP, it is connectionless, because it does not establish a connection with the remote computer. Instead, it is what is known as a best effort service, in that it will do whatever is possible to ensure that it works correctly, but its reliability is not guaranteed. The Internet Protocol determines the format for the packet headers, including the IP addresses of both the originating and the recipient devices.

IP Addresses:

A domain name is the web address that you normally type into a web browser. That name identifies one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used in URLs to identify particular Web pages. For example, in the URL http://www.pcwebopedia.com/index.html, the domain name is pcwebopedia.com. Every domain name has a suffix that indicates which top level domain (TLD) it belongs to. There are only a limited number of such domains, For example:

• .gov – Government agencies
• .edu – Educational institutions
• .org – Organizations (nonprofit)
• .com – Commercial Business
• .net – Network organizations

Because the Internet is based on IP addresses, not domain names, every Web server requires a Domain Name System (DNS) server to translate domain names into IP addresses. IP Addresses are the identifiers that are used to differentiate between computers and other devices that are connected to a network. Each device must have a different IP address, so that there are no problems of mistaken identity within the network. IP addresses consist of 32 bits that are divided in four 8 bit octets which are separated by dots. Part of the IP address identifies the network, and the remainder of the IP address identifies the individual computers on the network.

There are both public and private IP addresses. Private IP addresses are used by private networks that have no connection with outside networks. IP addresses within a private network should not be duplicated within that network, but computers on two different – but unconnected – private networks could have duplicated IP addresses. The IP addresses that are defined by IANA, the Internet Assigned Numbers Authority, as being available for private networks are:

• 10.0.0.0 through 10.255.255.255
• 172.16.0.0 through 172.31.255.255
• 192.168.0.0. through 192.168.255.255

IP addresses are divided into classes based on what portion of the address is used to identify the network and what portion is used to identify the individual computers. Depending on the size assigned to each part, more devices will be allowed within the
network, or more networks will be allowed. The existing classes are:

PROTOCOLS

• Class A: The first bit is always zero, so this class includes the addresses between 0.0.0.0 and 126.255.255.255. Note: the addresses of 127.x.x.x are reserved for the services of loopback or localhost.

• Class B: The first two bits of the first octet are ’10′, so this class includes the addresses between 128.0.0.0 and 191.255.255.255.

• Class C: The first three bits of the first octet are ’110′, so this class includes the addresses between 192.0.0.0 and 223.255.255.255.

• Class D: The first four bits of the first octet are ’1110′, so this class includes the addresses between 224.0.0.0 and 239.255.255.255. These addresses are reserved for group multicast implementations.

• The remaining addresses are used for experimentation or for possible future allocations.

At this time, the classes are not used to differentiate between the part of the address used to identify the network and the part used to identify the individual devices. Instead, a mask is used. In the mask, a ’1′ binary bit represents the part containing the network identification and a ’0′ binary bit represents the part that identifies the individual devices. Therefore, to identify a
device, in addition to the IP address, it is necessary to specify a network mask:

• IP: 172.16.1.20
• Mask: 255.255.255.0
• IP addresses 127.x.x.x are reserved to be used as loopback or local host addresses, that is, they refer directly back to the local computer. Every computer has a local host address of 127.0.0.1, therefore that address cannot be used to identify different devices. There are also other addresses that cannot be used. These are the network address and the broadcast address.

The network address is an address in which the part of the address which normally identifies the device is all zeros. This address cannot be used, because it identifies a network and can never be used to identify a specific device.

• IP: 172.16.1.0
• Mask: 255.255.255.0

The broadcast address is an address in which the part of the address which normally identifies the device is all ones. This address cannot be used to identify a specific device, because it is the address that is used to send information to all of the computers that belong to the specified network.

• IP: 172.16.1.255
• Mask: 255.255.255.0

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