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Classful IP Addressing (IPv4)

cisco-ccna-subnetting-02IP addressing is among the most important topics in any examination of TCP/IP. The IP address is a 32 bit binary identifier which, when configured correctly, enables each machine on an IP network to be uniquely identified. It is used to allow communication with any specific device on the network.

An IP address is defined in software and is configured dynamically as needed by software whether controlled by a human or a software process (as opposed to a MAC address which is a permanent, hard coded hardware address which cannot be easily changed). IP addressing was designed to allow media independent communication between any two hosts on the same, or different, IP networks.

Terminology

As a precursor to looking at IP Addressing in some detail, lets define some basic terminology.

Byte - A byte is a unit of binary information in that most commonly consists of eight bits. In the course of this post, the term Octet will also be used to represent one and the same thing.

IP Address - An IP address is a 32 bit binary number which represents, when assigned to a network device, its unique Network Layer (Layer 3) address. IP addresses are commonly described in Dotted Decimal notation for ease of human readability. Dotted Decimal notation is the conventional way of describing an IP address eg. 192.168.1.1 and is formed by separating the 32 bit IP address into 4 x 8-bit Octets, converting each Octet into a decimal number between 0 and 255 and separating each of these Octets with a dot. An IP Address is also frequently referred to as a Network Address and the terms can be used interchangeably however IP Address is by far the most common.

Broadcast Address - On any IP Network, the Broadcast Address is the address used to send to all hosts which are members of and connected to the IP Network.

IP Addressing

As mentioned previously, an IP address is made up of 32 binary bits. It is extremely important to always bear this fact in mind when working with IP addresses as failing to do so can significantly impair ones ability to fully understand and manipulate the IP addressing system as required.
IP addresses are commonly described in one of three ways -

    1. Dotted Decimal (As described above)

 

    1. Binary (As a 32 bit binary number)

 

    1. Hexadecimal (Rarely used but can be seen when addresses are stored within programs or during packet analysis)



One important aspect of an IP address is that it is an hierarchical address. This has a number of advantages not least of which is the fact that it enables addresses to be aggregated together which greatly simplifies the mechanisms which are used to route traffic around the Internet.
In IPv4 there are 4.3 billion IP addresses available in theory and without this mechanism for route aggregation it would be necessary for Internet routers to know the location of each one of these connected devices.
The hierarchical system used by IPv4 is one which separates the IP address into two components, namely a network part and a host part.
In practice this "two component" system is further split down as the host part is frequently subdivided into even smaller subnetworks. In this post however we will limit or discussion to the "two component" system.

This term, "subnetwork", (often abbreviated to subnet) is one which is used frequently within the network engineering community to such an extent that it has become part of the jargon of the trade. This has only served to enhance its status as a term which has a great deal of complexity behind it but it is actually extremely simple. A subnetwork (subnet) is any subdivision of a larger network. It really is as simple as that.
The Two Component System / Network part and Host part

In order to make IP addresses hierarchical, a two component system has been created. This system splits the IP address into two parts known as The Network Part and The Host Part. This can be likened to a telephone number where (typically) the first 4 or 5 digits represent the town or city and the subsequent 6 or 7 digits represent the individual line.
The designers of this hierarchical addressing scheme created 5 classes of IP address by splitting up the full range of 4.3 billion addresses in a logical way. These 5 classes (or subdivisions) are known as Class A, B, C, D, and E networks.
For the purposes of this post we shall concern ourselves primarily with classes A, B and C however I shall briefly introduce each of the classes in the following section.
The 5 Network Classes
The image below depicts the 5 classes of IP Network as well as some of the basic features associated with each.

Screenshot_2

Class A - Class A networks were designed for use in networks which needed to accommodate a very large number of hosts.
As can be seen from the diagram, the first bit in a Class A address is always 0.
In each of network classes A, B and C, we can also see that the addresses are split into two parts, namely Network and Hosts.
These parts can be likened to the two parts of the telephone number described earlier.
The Network part is like the city code and the Host part is like the rest of the telephone number.
As you can see from the image, the division between the Network and Host part is set after the 8th bit. This means that we have 7 bits available to represent different Networks and 24 bits available to represent the individual hosts within each of the Class A networks.
It is clear therefore that, since the first bit must always be 0, the lowest network address available is 00000000.X.X.X (0 in decimal) and the highest network address available is 01111111.X.X.X (127 in decimal).
It would seem therefore that the range of addresses available to Class A networks is 0.X.X.X up to 127.X.X.X (Where X represents the Host part) but I shall demonstrate later that the 0 and 127 networks are reserved therefore the Class A address range runs from 1.X.X.X to 126.X.X.X in practice.

Class B - In Class B networks, the split between the network part and the host part happens after the 16th bit.
In any Class B network address the first two bits must always be set to 10. This leaves 14 bits to define the network number and allows addresses to range from 10000000.00000000.X.X up to 10111111.11111111.X.X .
These binary addresses equate in decimal to the first two Octets of Class B addresses ranging from 128.0.X.X up to 191.255.X.X
Class C - The pattern now emerging is that Class C addresses use the first 3 Octets to define the Network part of their addresses. Again, as with Class A and B networks some bits are permanently defined and in the case of Class C network addresses, the first 3 bits are always set to 110.
This means that we have 21 bits available to define the network part of Class C network addresses ranging (in binary) from 11000000.00000000.00000000.X up to 11011111.11111111.11111111.X which in decimal equates to 192.0.0.X up to 223.255.255.X

Class D - Class D (224-239) is reserved for Multicast Addressing and a post based explicitly on this addressing will be published ASAP and linked to from here. Class D addressing is beyond the scope of this post however if required please click this link for more detail. Class D Networks and IP Multicasting. .

Class E - Class E (240-255) is reserved for scientific experimentation and research and if any subsequent posts on this blog examine Class E networks, they will be linked to from here.

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