The TCP/IP suite was developed by the Department of Defence (DOD) before the OSI Model, and consequently it does not map directly onto the OSI Model. However, it does map closely to the layers of the OSI Model.
The TCP/IP suite is based on the DOD Model and has four layers:
- Application Layer - This corresponds to the top three (application related) layers of the OSI Model.
- Host-to-Host Layer - This corresponds to the Transport layer of the OSI Model.
- Internet Layer - This corresponds to the network layer and the MAC part of the data link layer of the OSI Model.
- Network Access Layer - This corresponds to the LLC part of the data ink layer of the OSI Model.
As mentioned before, the TCP/IP suite is made up of several protocols. These are given below:
- HTTP - The Hypertext Transfer Protocol defines the message format and transmission used by web servers and web browsers.
- Telnet - This is a terminal emulation protocol that allows you to connect a local computer to a remote computer or vice-versa.
- SMTP - This is the Simple Mail Transfer Protocol and it provides email services on the Internet and IP networks.
- TCP - This is a connection-oriented transport protocol.
- UDP - The User Datagram Protocol is a connectionless transport protocol.
- IP - This is the basis for all addressing on TCP/IP networks and provides a connectionless-oriented network layer protocol.
- ARP - The Address Resolution Protocol maps IP addresses to MAC hardware addresses.
- RIP - The Routing Information Protocol is a routing protocol used by routers to determine the best path for packets travelling on an internetwork.
In order that each computing device on a network be uniquely identified, the TCP/IP protocol uses and addressing system that is 32 bits (4 bytes) long, represented in dotted decimal format, to represent each device on the network. This system divides the 32 bits (4 bytes) into four groups of 8 bits (one byte) each, separated by dots (periods), for example 188.8.131.52.
This addressing system is further sub-divided into three main classes that can be used in networks. There are however, five in total:
- Class A - Addresses in this range begin with the numbers 1 – 126 in the first byte of the address, and are used for companies with large networks.
- Class B - Addresses in this range begin with the numbers 128 – 191 in the first byte of the address, and are used for companies with medium sized networks.
- Class C - Addresses in this range begin with the numbers 192 – 223 in the first byte of the address, and are used for companies or individuals with small networks.
- Class D - Addresses in this range begin with the numbers 224 – 239 in the first byte of the address, and are not used as network addresses. They are reserved for multicasting.
- Class E - Addresses in this range begin with the numbers 240 – 255 in the first byte of the address, and are not used as network addresses. They are reserved.
IP addresses have two parts: the Network ID and the Host ID. The Network ID part of the address identifies the network, while the Host ID part of the address identifies the device on the network. Each of the classes of IP addresses have a finite number of network addresses and hosts:
- Class A - 126 networks and 16 million hosts.
- Class B - 16,000 networks and 65,000 hosts.
- Class C - 2 Million networks and 254 hosts.
Private IP Addressing
There are certain addresses that have been reserved, within each class of IP addresses, for private addressing. That is, if you wish to set up your own private network there are addresses that you can use that will not conflict with any address that has been assigned to computers on the Internet. It is therefore recommended that if you are going to set up a private network that is going to be connected to the Internet, that you use and address in the following range:
- Class A - 10.0.0.1 to 10.255.255.254, with a subnet mask of 255.0.0.0. This is in effect any address that looks like 10.x.x.x.
- Class B - 172.16.0.1 to 172.31.255.254, with a subnet mask of 255.255.0.0. This is in effect any address in the range 172.16.x.x to 172.31.x.x.
- Class C - 192.168.0.1 to 192.168.255.254, with a subnet mask of 255.255.255.0. This is in effect any address that looks like 192.168.x.x.