Basic Knowledge of TCP/IP – Part 1

TCP/IP suite of protocols

The TCP/IP suite is a set of communications protocols used on computer networks today, most notably on the Internet. It provides an end-to-end connectivity by specifying how data should be packetized, addressed, transmitted, routed and received on a TCP/IP network. This functionality is organized into four abstraction layersand each protocol in the suite resides in a particular layer.

The TCP/IP suite is named after its most important protocols, the Transmission Control Protocol (TCP) and theInternet Protocol (IP). Some of the protocols included in the TCP/IP suite are:

  • ARP (Address Resolution Protocol) – used to convert an IP address to a MAC address.
  • IP (Internet Protocol) – used to deliver packets from the source host to the destination host based on the IP addresses.
  • ICMP (Internet Control Message Protocol) – used to detects and reports network error conditions. Used in ping.
  • TCP (Transmission Control Protocol) – a connection-oriented protocol that enables reliable data transfer between two computers.
  • UDP (User Datagram Protocol) – a connectionless protocol for data transfer. Since a session is not created before the data transfer, there is no guarantee of data delivery.
  • FTP (File Transfer Protocol) – used for file transfers from one host to another.
  • Telnet (Telecommunications Network) – used to connect and issue commands on a remote computer.
  • DNS (Domain Name System) – used for host names to the IP address resolution.
  • HTTP (Hypertext Transfer Protocol) – used to transfer files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web.

The following table shows which protocols reside on which layer of the TCP/IP model:+

tcp ip protocol layers

What is an IP address?

An IP address is a 32-bit number assigned to each host on a network. Each device that wants to communicate with other devices on a TCP/IP network needs to have an IP address configured. For example, in order to access the Internet, your computer will need to have an IP address assigned (usually obtained by your router from your ISP).

An IP address is usually represented in dot-decimal notation, consisting of four-decimal numbers seperated by periods (e.g. The first part of the address usually represents a network the device is on (e.g., while the last part of the address identifies the host device (e.g.

An IP address is a software (logical) address, not a hardware address hard-coded on a NIC like a MAC address.

An IP address can be configured manually or be obtained from a DHCP server on your network. To find out your IP address in Windows, open the Command Prompt (Start > Run > cmd):

open cmd+

Type the ipconfig command. You should see a field named IPv4 Address:

windows ipconfig command+

To find out your IP address in Linux, use the ifconfig command. The field inet addr represents an IP address:

ifconfig ip address+

The term IP address is usually used for IPv4, which is the fourth version of the IP protocol. A newer version,IPv6, exists, and uses 128-bit addressing.

Private IP addresses

The original design of the Internet specified that every host on every network should have a real routable IP address. An organization that wanted to access the Internet would complete some paperwork, describing its internal network and the number of hosts on it. The organization would then receive a number of IP addresses, according to its needs. But there was one huge problem with this concept – if every host on every network in the world was required to have an unique IP address, we would have run out of IP addresses to hand out a long time ago!

The concept of private IP addressing was developed to address the IP address exhaustion problem. The private IP addresses can be used on the private network of any organization in the world and are not globally unique. Internet routers are configured to discard any packets coming from the private IP address ranges, so these addresses are not routable on the Internet.

Consider the following network

private addressing explained

In the picture above you can see that two organizations use the same private IP network ( inside their respective internal networks. Because private IP addresses are not globally unique, both organizations can use private IP addresses from the same range. To access the Internet, the organizations can use a technology calledNetwork Address Translation (NAT), which we will describe in the later lessons.

There are three ranges of addresses that can be used in a private network:

  • –
  • –
  • –

IP address classes

IP addresses are divided into five classes that are identified by the value of the first octet (the first decimal number). The system of IP address classes was developed for the purpose of Internet IP addresses assignment. The classes created were based on the network size. For example, for the small number of networks with a very large number of hosts, the Class A was created. The Class C was created for the numerous networks with the small number of hosts.

The IP address classes are:

  • Class A, 0-127 – for example, For large networks with many devices.
  • Class B, 128-191 – for example, For medium-sized networks.
  • Class C, 192-223 – for example, For small networks with the small number of hosts.
  • Class D, 224-239 – for example, For multicast addresses.
  • Class E, 240-255 – for example, Experimental.

Reserved addresses (used for special purposes):

  • – used to communicate with the network the device is on.
  • – loopback addresses.
  • – link-local addresses (APIPA).

An IP address consists of 32 bits. These bits are divided into two parts:

  • network bits – identify a particular network.
  • host bits – identify a host on the network.

For the IP addresses from Class A, the first 8 bits (the first decimal number) represent the network part, while the remaining 24 bits represent the host part. For Class B, the first 16 bits (the first two numbers) represent the network part, while the remaining 16 bits represent the host part. For Class C, the first 24 bits represent the network part, while the remaining 8 bits represent the host part. For example, consider the following IP addresses: – because this is a Class A address, the first number (10) represents the network part, while the remainder of the address represents the host part (50.120.7). This means that, in order for devices to be on the same network, the first number of their IP addresses has to be the same for both devices. In this case, a device with the IP address of is on the same network as the device with the IP address listed above. The device with the IP address is not on the same network, because the first number of its IP address is different. – because this is a Class B address, the first two numbers (172.16) represents the network part, while the remainder of the address represents the host part (55.13). The device with the IP address of is on the same network, while a device with the IP address of isn’t.

The system of network address ranges described here is generally bypassed today by use of the Classless Inter-Domain Routing (CIDR) addressing.

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