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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

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 public 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