Publish Date: June 17, 2015

IP Addressing

An IP address is a numeric identifier assigned to each device on an IP network. It consists of 32 bits which are divided into four sections, referred to as octets or bytes, each containing 1 byte (8 bits). IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in.
Before we get into the more complicated aspects of IP addressing, you need to understand some of the basics. First I am going to explain some fundamentals of IP addressing and its terminology.

Important Terminology

Bit: A bit is one digit, either a 1 or a 0.

Byte: A byte is 8 bits.

Octet: An octet, made up of 8 bits, is just an ordinary 8-bit binary number.

Network address: This is the designation used in routing to send packets to a remote network. Examples:,

Broadcast address: The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples:, which is any network, all nodes, which broadcasts to all subnets and hosts on network

Network Addressing: The network address or network number uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. For example in the IP address, 172.16 is the network address.

Subnet Mask: A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address Assuming this is part of a Class C network, the first three numbers (192.168.1) represent the Class B network address, and the last one number (10) identify a particular host on this network.

Broadcast Domain: A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

Collision Domain: A part of a network where packet collisions can occur. A collision occurs when two devices send a packet at the same time on the shared network segment. The packets collide and both devices must send the packets again, which reduces network efficiency. Collisions are often in a hub environment, because each port on a hub is in the same collision domain but each port on a bridge or a switch is in a separate collision domain.

Layer-2 Broadcasts: Layer 2 broadcast traffic stays within a local area network (LAN) boundary; known as the broadcast domain. Layer 2 broadcast traffic is sent to the broadcast domain using a MAC address of FF:FF:FF:FF:FF:FF. Every device in the broadcast domain recognizes this MAC address and passes the broadcast traffic on to other devices in the broadcast domain, if applicable.

Layer-3 Broadcasts: Layer 3 broadcast traffic, however, is sent to all devices in a network using a broadcast network address. For example, if your network address is, the broadcast network address is In this case, only devices that belong to the network receive the Layer 3 broadcast traffic. Devices that do not belong to this network drop the traffic.

Unicast: This is an address for a single interface, and these are used to send packets to a single destination host.

Multicast: These are packets sent from a single source and transmitted to many devices on different networks. Generally referred to as “one-to-many”.

IP Address Classes

The designers of the internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank Class A network. At the other extreme is the Class C network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the Class B network. Subdividing an IP address into a network and node address is determined by the class designation of one’s network.

There are 5 classes of IPv4 Addressing scheme. All the 5 classes are identified by the first octet of IP Address and first octet referred here is the left most of all.

Class A
The first bit of the first byte in a Class A network address must always be off or 0. This means a Class A address must be between 0 and 127 in the first byte.
Consider the following network address:
If we turn the other 7 bits all off and then turn them all on, we will find the Class A range of network addresses:
00000000 = 0
01111111 = 127
But according to designers, 127 is not a valid address; it is reserved for some specific use; I will discuss it later under reserved addresses.

So, a Class A network range between between 0 to 126.

Class B
In a Class B network, the first bit of the first octet must always be turned on but the second bit must always be turned off. If you turn the other 6 bits all off and then all on, you will find the range for a Class B network:
10000000 = 128
10111111 = 191
So, a Class B network range between 128 to 191.

Class C
In Class C networks, first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process, convert from binary to decimal to find the range:
11000000 = 192
11011111 = 223
So, a Class C network range is between 192 to 223.
Class D
Class D range is between 224 to 239 and it is reserved to be used for multicast addresses.

Class E
Class E network range is between 240 to 255 and this is reserved for scientific purposes. That is why we are not going to discuss Class D and E here.

Private IP Addresses

The people who created the IP addressing scheme also created private IP addresses. These addresses can be used to create private networks and they are not routable through the Internet. That is why these addresses are referred to as non-routable addresses. RFC 1918 defines the following list of private addresses reserved for internal use:

Address Class

First Address

Last Address

Number of Addresses







Reserved IP Addresses

Below mentioned IP addresses are reserved for specific function and they can not be used in production networks.

Address Function
Network address of all 0 This network or segment
Network address of all 1 All networks
Network Reserved for loopback testing. Designates the local node and allows the node to send a test packet to itself without generating network traffic
Node address of all 0 Network address or any host on a specified network.
Node address of all 1 All nodes on the specified network; for example, means “all nodes” on network 128.2 (Class B Address)
Entire IP address set to all 0 Used by Cisco routers to designate the default route. Also mean “any network.”
Entire IP address set to all 1 ( Broadcast to all nodes on the current network; referred as limited broadcast.

Internet Protocol Versions

Two versions of the Internet Protocol (IP) are in use: IP Version 4 and IP Version 6. Each version defines an IP address differently. Because of its prevalence, the generic term IP address typically still refers to the addresses defined by IPv4.

In IPv4 an address consists of 32 bits which limits the address space to 4294967296 (232) possible unique addresses. IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses(~270 million addresses). Because every device which want to communicate with internet need to have a unique routable IP address and rapid grow in in internet usage over a decade resulted in IPv4 address exhaustion. To overcome the shortage of IP address space, IP protocol is redesigned. This new generation of the Internet Protocol, intended to replace IPv4 on the Internet is known as  named Internet Protocol Version 6 (IPv6). The address size was increased from 32 to 128 bits or 16 octets. This, even with a generous assignment of network blocks, is deemed sufficient for the foreseeable future. Mathematically, the new address space provides the potential for a maximum of 2128, or about 3.403×1038addresses.


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