Introduction to TCP/IP model and IP addressing

The term “IP address” is probably not a new term and many may already be familiar with it as an address assigned to your computer or mobile device when connected to a network.

However, let’s go a little deeper than the definition and try to understand the protocol itself, its versions, types and how it works.

TCP/IP or the Internet Protocol Suite

First of all, IP stands for Internet Protocol which is a set of standards and requirements for transmitting data packets across networks. The Internet Protocol (IP) is part of the Internet layer of the TCP/IP suite. In the OSI model, IP would be considered a part of the network layer.

Protocols are nothing but a standard set of rules and procedures that determines how different systems will communicate across networks. Two different systems that use the same protocol can communicate and understand each other just like how different people can communicate with each other using the same language.

The OSI model is a reference/logical model which was designed to describe the functions of the communication system in the form of seven layers, with one or more protocols associated with each layer. TCP/IP is rather a more concise and practical version of the OSI model, used in the internet and similar computer networks. It is has been named as “TCP/IP” because of the two most important protocols in the suite, that is, the Transmission Control Protocol (TCP) and the Internet Protocol (IP).

TCP/IP combines some layers of the OSI model into a single layer, resulting in a total of four layers, namely:

1. Network Access/Link Layer: This is a combination of the Data Link Layer and Physical Layer of the OSI model. It is responsible for hardware addressing and the physical transmission of data. In other words, the protocols working in this layer defines how data should be sent between two devices on a network.
2. Internet Layer:This layer corresponds to the network layer of the OSI model. It defines the protocols which are responsible for logical transmission or delivery of data packets over the entire network. Some of the main protocols working in this layer are IP, ICMP, ARP etc.
3. Host-to-Host/Transport Layer: This layer corresponds to the transport layer of the OSI model. It is responsible for end-to-end communication , flow control and error-free delivery of the data packets. The main protocols of this layer are TCP and UDP.
4. Application Layer: This layer performs the functions of the top three layers of the OSI model, that is, Application, Presentation and Session Layer. It is responsible for allowing the user to interact with an application. Some of the protocols present in this layer are HTTP, FTP, SSH, SMTP, etc.

The protocols within the TCP/IP suite work together to break down the data passed from the application layer into packets that can be moved along a network. They work with other protocols to transmit the data to the destination computer and then reassemble the data back into a form that the application layer can understand and process.

IP is an internet or network layer protocol which provides data transmission or routing services. It is a connectionless protocol which envelops data passed to it from the transport layer. The IP protocol addresses the datagram with the source and destination IP addresses.

There are two main protocols that work in the transport layer, that is, TCP and UDP. TCP is known to be a  connection-oriented protocol as it ensures that each and every packet is delivered to the destination through packet sequencing and also makes sure to resend corrupted or lost packets. UDP is another transport layer protocol which is known to be a connectionless protocol as it neither has packet sequencing nor the ability to know if the packets were received properly or were lost/dropped during the transmission.

While IP uses IP addresses for routing data packets, the transport layer protocols uses  ports to communicate with the other layers. Therefore each message contains source and destination IP Addresses along with source and destination ports and is how the data packets know where to go (by the IP Address) and how to communicate with the right service (by the Port number) on the destination.

IP Addressing

An IP Address, or Internet Protocol Address is an identifier that uniquely identifies a device, also known as a host or node, which is connected to an IP network. Each node on a network has an IP address, which helps in communicating with the other nodes or devices on the network. IP addresses are binary numbers that are usually represented in decimal (IPv4) or hexadecimal (IPv6) form to make it more user friendly.

There are currently two standards or versions of IP, namely, IPv4 and IPv6.

IPv6

IPv6 was developed by the Internet Engineering Task Force (IETF). It increased the available address space and allowed for 2¹²⁸ addresses. IPv8 uses 128 binary bits to create a single unique address which is represented by eight groups of hexadecimal (ie, alphanumeric) numbers separated by colons, instead of a dot.

For example:

2001:cdba:0000:0000:0000:0000:3257:9652

Note: For groups of numbers that contain all zeroes, they  are often omitted to save space, leaving a colon separator to mark the gap. Therefore, the above address can also be written as:

2001:cdba::3257:9652

IPv4

When IPv4 was first introduced, there were not many internet users and it was built for only 2³² addresses, which is about 4.3 billion unique addresses. However, the current increase in the demand of addresses led to a possible shortage and hence, a new version of IP known as IPv6 was developed to increase the supply.

IPv4 uses 32 binary bits to create a single unique address and is usually represented as four decimal numbers separated by dots. Each of the four decimal number represents an eight digit binary number known as an octet or a byte.

For example: this 32 bits binary number “10101000110101001110001011001100” is divided into 4 octets having 8 bits each and these octets are then separated by a dot “.” as –

10101000.11010100.11100010.11001100

Lastly, these 4 octets are then converted into their corresponding decimal number as –

168.212.226.204

As mentioned before, each decimal represents an eight-digit binary number. So this means that each number could range from 00000000 to 11111111 in binary, or 0 to 255 in decimal form. In other words, the IPv4 addresses could range from  0.0.0.0 to 255.255.255.255. Regardless, there are some certain addresses in that range which have been reserved for specific purposes such as:

• 0.0.0.0: This address represents the default network, which is the abstract concept of just being connected to a TCP/IP network.

• 255.255.255.255: This address is reserved for broadcasting, or sending messages to all computers on the network.

• 127.0.0.1: This address is known as the loopback address, which is your computer’s way of identifying itself, whether or not it has an assigned IP address.

Furthermore, IPv4 addresses are composed of two parts. The first part in the address known as the network addressidentifies the network, while the latter numbers identifies the specific interface on that network and is known the host address. All the hosts connected to the same network will have the same network address (first numbers of the IP address) but different host addresses. In other words, the host address identifies every individual node or system, and the network address is the identity of the network they are all connected to; therefore, it is the same for each one of them.

A subnet maskdetermines where the network part of an IP address ends, and the host portion of the address begins.When written in binary form, any bit set to one means the corresponding bit in the IP address is part of the network address. All the bits set to zero mark the corresponding bits in the IP address as part of the host address.

For example, if thenumber of bits denoting the network address is 16, then the subnet mask can be written as:

1111 1111.1111 1111.0000 0000.0000 0000

(since network address bits are set to 1 and host address bits set to 0)

Or

255.255.0.0 (since 1111 1111 in binary form is equivalent to 255 in decimal form)

IPv4 Classes

Class A:In a Class A address, the first octet (8 bits) in binary or the first decimal, is reserved as the network part of the address, with the remaining part of the address, that is, 3 octets or 3 decimal numbers, being the host part of the address. Class A addresses are for big organisations.

Class B:In a Class B address, the first 2 octets (16 bits) or the first 2 numbers in decimal form are the network part of the address and the remaining 2 decimal numbers or octets are the host part of the address. It is usually used by medium sized companies.

Class C:In case of Class C, the first three octets (24 bits) in binary or the first 3 decimal numbers specify the network address and the remainder is the host address. Class C addresses are meant for small companies.

Class D:Class D addresses are used for multicasting applications. Unlike the previous classes, the Class D is not used for “normal” networking operations.

Class E:Class E networks are reserved for future/experimental use.

Private Addresses

There are a few Private IPv4 addresses which have been reserved for internal LAN address use. However, these addresses can only be used within the boundaries of a company or its internal network and cannot be used to communicate over the Internet because they will not be properly routed. The following lists current private IP address ranges:

Static and Dynamic IP Addresses

IP addresses also can either be static or dynamic.Static IP addresses do not change automatically and are permanently assigned to you by your Internet Service Provider (ISP). These addresses are usually assigned to server hosting websites or huge public organisations requiring the same IP address every time. They are more stable but at the same time, costlier.

Dynamic IP addresses, on the other hand, are subject to change. These addresses are temporary IP addresses which are actually borrowed from a pool of IP addresses, shared over various systems and devices. Dynamic addresses are assigned, as needed, by Dynamic Host Configuration Protocol (DHCP) servers of your ISP. This allows ISPs to provide their subscribers an IP address that might change if needed.