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Lesson 4 TCP/IP addresses and classifications
ObjectiveDescribe TCP/IP Addresses and how they are classified.

TCP/IP Addresses and Classifications

Here is a breakdown of the characteristics and classification of TCP/IP addresses:
Characteristics of TCP/IP Addresses
  • Hierarchical: TCP/IP addresses (specifically IPv4) have a hierarchical structure, divided into a network portion and a host portion. This allows for efficient routing of data across networks.
  • 32-bit Binary (IPv4): IPv4 addresses are 32 bits long, typically represented in dotted-decimal notation (e.g., Each of the four decimal numbers represents 8 bits (one octet).
  • 128-bit Binary (IPv6): IPv6 addresses are 128 bits long and generally written in a hexadecimal format with colon separators (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334) They were created to address the exhaustion of IPv4 addresses.
  • Uniqueness: Within a specific network, a TCP/IP address must be unique to a device for proper communication.

Classification of TCP/IP Addresses
  • Unicast: A single sender to a single receiver. This is the most common type used for everyday internet communications.
  • Multicast: A single sender to a specific group of receivers. Used for things like video streaming or online conferencing.
  • Broadcast: A single sender to all devices on a network. Used for purposes like device discovery or network announcements.

IPv4 Address Classes (Historical) Historically, IPv4 addresses were further divided into classes:
  • Class A:
    • First octet range: 1 - 126
    • Large networks (millions of hosts)
  • Class B:
    • First octet range: 128 - 191
    • Medium-sized networks (thousands of hosts)
  • Class C:
    • First octet range: 192 - 223
    • Small networks (hundreds of hosts)
  • Class D:
    • Reserved for multicast addressing.
  • Class E:
    • Reserved for experimental purposes.
Modern Classless Addressing The class-based system is now largely obsolete due to its inefficiencies. Modern addressing uses Classless Inter-Domain Routing (CIDR), which allows for more flexible allocation of IP address blocks.

TCP/IP Addresses

A TCP/IP address is a unique address that identifies a computer that is connected to a network. A network can be a LAN, or it might be your ISP (internet service provider). An IP address consists of a 32-bit number represented as a four-part number with each part separated by a period (like x. x. x. x). Each part is called an octet and has a value from 0 to 255. Octets are used to identify the network address and the local host address and are always shown as decimal values (even though they consist of 32 binary values). The diagram below illustrates the elements of the IP addressing scheme. In this diagram, the Network Number is shown as four octets. However, it can be any number desired.
The network number is shown as four octets
The network number is shown as four octets

For example, if an IP address were, this is how that address actually looks in binary:
Binary representation of IP Address
Binary representation of IP Address

Address Classifications

To accommodate an efficient numbering system, TCP/IP addresses are divided into five classes (A, B, C, D, and E). Classes are similar to the telephone system's area code concept. Instead of assigning each telephone a unique number, telephones are grouped by area code. Grouping in this way reduces the total number of digits required to identify a single phone (or in the case of TCP/IP, a single computer). Instead of organizing users by area code, TCP/IP organizes users by network. The five classes of networks essentially specify the maximum quantity of networks possible for a particular company. An address's class can be identified by the first octet.
  Octet 1 Octet 2 Octet 3 Octet 4
Class A 0      
Class B 10      
Class C 110      
Class D 1110      

The table above defines the first octets for each class and describes how the octets are read to identify the network address and the local host address. Notice that Class A dedicates its first bit position to a 0. Class B sets its first bit to a 1 and its second bit to a 0. The other classes follow suit. Routers are designed to specifically read these patterns to determine the address class. Once known, the remaining bits making up the 32-bit address can be read to determine additional information about the address.

Making sense of the Numbers

For a real world example, consider your phone number again. It starts with an area code, then a local exchange. For example, if your phone number is 415-555-1010, it tells the telephone company that you are in a particular region (based on 415), in a particular neighborhood (based on 555), and on a particular street (based on 1010). Similarly, networks use identifiable IP addresses like the one illustrated below.
IP Address consisting of 1) Network Number and 2) Host Number
IP Address consisting of 1) Network Number and 2) Host Number
If your IP address was, your PC would be on a Class-A network. Additionally, assuming a simplistic network number system is being used, that particular network would be Network number1, and your PC would be node number 3.
InterNIC is the agency that assigns

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Understanding Public and Private IP Addresses

As we continue to navigate the ever-evolving digital landscape, it's essential to understand the difference between public and private IP addresses. In this article, we will explain what these terms mean and how they impact your online experience.
  1. What is a Public IP Address? A public IP address is a unique identifier that is assigned to a computing device and is accessible over the Internet. Just like a postal address is used to deliver physical mail to your home, a public IP address is necessary for data to be sent and received over the Internet. To find your public IP address, you can visit a website that offers this service, such as Your public IP address is assigned to your router by your Internet Service Provider (ISP) and can be used to identify your location and the devices connected to your network.
  2. What is a Private IP Address? Unlike public IP addresses, private IP addresses are used to assign devices within a private network without exposing them to the Internet. For instance, if you have multiple devices within your home network, you may want to assign private IP addresses to each device. In this scenario, your router is assigned the public IP address, and each device connected to your network (via wired or wireless connection) is assigned a unique private IP address. The router assigns these addresses via the Dynamic Host Configuration Protocol (DHCP).

Reserved IP Blocks for Private Use

To allow organizations to freely assign private IP addresses, the Network Information Center (InterNIC) has reserved certain address blocks for private use. These address blocks are not publicly routable and are only used within private networks. The following IP address blocks are reserved for private IP addresses:
The following IP blocks are still reserved for private use as defined in RFC 1918 by the Internet Assigned Numbers Authority (IANA). Here's the breakdown:
  1. - (10/8 prefix): This block provides a massive number of addresses for large private networks.
  2. - (172.16/12 prefix): This block is intended for medium-sized private networks.
  3. - (192.168/16 prefix): This block is commonly used for home networks and smaller private networks.

Why Private IP Addresses?
  • Conserve Public IP Space: With the explosive growth of the internet, it's essential to preserve public IP addresses. Private IP addresses allow countless devices to exist on internal networks without needing globally unique public addresses.
  • Internal Communication: Devices within a private network can communicate using these addresses without traversing the public internet.
  • Security: Private IP addresses are not directly reachable from the public internet, adding a layer of inherent security.

Important Note: Devices using private IP addresses usually communicate with the public internet through Network Address Translation (NAT) performed by routers or gateways.
By using private IP addresses, organizations can efficiently manage their internal networks while maintaining security and privacy.

More on Public IP Addresses

Public IP addresses are addresses recognized across the Internet. When you register with InterNIC, they issue you a public IP address, which ensures that the address is unique. However, IP addresses, like phone numbers for telephones, are becoming scarce. As a result, networks are using private IP addresses, which are valid only on a local intranet. It is never used on the Internet. The technique used for private addressing is called subnetting, and will be discussed in the next lesson. It is not necessary to register private IP addresses.

Private Networks behind Router

Small organizations with limited Internet gateway access can place their networks behind a router that keeps the addresses private and use the private IP addresses that are designed for this purpose. Private IP addresses are not recognized as valid on the Internet. The source address of IP packets originating on the private network is modified to conform to the public IP address. Conversely, IP packets from the outside are readdressed with the private destination address when they pass through the router from the outside into the private network. There are two difficulties with this approach:
  1. Traffic passing through the router to the outside, for exampleWeb requests, must be carefully tracked by the router, so that responses from the outside are routed to the correct private address on the inside. For a small network, this is relatively manageable. The task becomes far more challenging when the number of privately addressed computers is large and traffic through the interface is substantial. The correct routing of e-mail is particularly challenging.
  2. An organization with multiple local area networks connected with a backbone network must configure the private networks in such a way that traffic between the various private local area networks can be managed successfully. One possible solution is to use a single private IP addressing scheme for all networks attached within the backbone, with translation at the edge router. Again, the problem of translation becomes large and difficult to manage.
In summary, understanding the difference between public and private IP addresses is crucial in today's digital age. A public IP address is required for devices to communicate over the Internet, while private IP addresses are used within private networks to assign unique addresses to devices without exposing them to the Internet. By reserving specific IP address blocks for private use, organizations can effectively manage their internal networks while ensuring security and privacy. So, the next time you are setting up your home or business network, remember the importance of public and private IP addresses.
In the next lesson, you will learn about the purpose of subnet masks.

TCP/IP Addresses Classifications

Click the link below to review TCP/IP addresses and their classifications.
TCP/IP Address Classifications

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