IP Addressing

IP Addressing

IP addressing is the role of the layer-3 ‘Network Layer’ in OSI Model or Layer-2 ‘Internet layer’ in TCP/IP model.

IPv4 Header

The IP header is used to support two key operations, routing and fragmentation.

  • Routing is the mechanism that allows traffic from a given network to be forwarded to other networks, since the data link layer represents a single network for which network boundaries exist.
  • Fragmentation refers to the breaking down of data into manageable blocks that can be transmitted over the network.
Figure-1: IPv4 Header


Classes of IP Addresses

The IPv4 addresses are classified into following five categories:

Figure-2: IP Address Classes

Class A for instance consist of 126 potential networks, each of which can support 254.

Class B for instance consist of 64534 potential networks, each of which can support 64534.

Class C address ranges allow for a much more balanced network that scales well to Ethernet networks, supplying just over 2 million potential networks, with each network capable of supporting around 256 addresses, of which 254 are assignable to hosts.

Class D is a range reserved for multicast, to a group, which require the member association to groups.

Class E is reserved for experimental purposes.

IP Address Types

Figure-3: IP Address Types

As the public IP in IPv4 are limited and cannot take care of the increasing demand of addresses an adhoc arrangement was made allowing few private IP to be reused within the enterprise.

However for outbound traffic a IP address translation be performed before data can reach the intended destination.

Other special addresses include :

  • a diagnostic range denoted by the network address
  • the first address represents any network
  • the last address represents a broadcast address for the IPv4 ( network, however the scope of any broadcast in IP is restricted to the boundaries of the local area network from which the broadcast is generated.

Subnet Mask

Application of the subnet mask to a given IP address enables:

  • identification of the network to which the host belongs.
  • identify the broadcast address for the network
  • Indicate the number of hosts that can be supported as part of the network range.

Such information provides the basis for effective network address planning.

How to find the network ID:

network address = (IP Address) . (Subnet Mask)

Let number of bits in Host address = n

Total number of hosts = 2n

Number of usage address =   2– 2


bits in subnet mask 24

therefore number of bits for hosts = 32-24 = 8

Total number of hosts = 28 = 256

Number of usage address =   28 – 2 = 254

VLSM Calculation

As a means of resolving the limitations of default subnet masks, the concept of variable length subnet masks are introduced, which enable a default subnet mask to be broken down into multiple sub-networks, which may be of a fixed length (a.k.a. fixed length subnet masks or FLSM) or of a variable length known commonly by the term VLSM. The implementation of such subnet masks consists of taking a default class based network and dividing the network through manipulation of the subnet mask.

To divide a network of 100 systems into 4 sub-groups:

25 hosts in each sub-group; each subgroup would need 5-bit host address leaving 27 bit network subnet mask

Figure-4: VLSM

Classless inter domain routing

Classless inter-domain routing was initially introduced as a solution to handle problems that were occurring as a result of the rapid growth of the Internet. The primary concerns were to the imminent exhaustion of the class B address space commonly adopted by mid-sized organizations for whom class C was inadequate and where class A was too vast, and management of the 65534 host addresses could be achieved through VLSM.

Figure-5: CIDR


The solution given involves transitioning to a classless addressing system in which classful boundaries were replaced with address prefixes. It uses the slash ‘/’ notation for subnet mask.

The result of CIDR has had far reaching effects and is understood to have effectively slowed the overall exhaustion rate of the IPv4 address space.


Internet is a network of network and i generally referred as a public network, where communication often is among different network.

Figure-6: Gateway

In the case where the intended network is different from the originating network, the packet must be forwarded to a gateway via which the packet is able to reach it’s intended destination.

In all networks, the gateway is a device that is capable of handling packets and making decisions as to how packets should be routed, in order to reach their intended destination.

Where networks are divided by a physical gateway, the interface IP address (in the same network or sub-network) via which that gateway can be reached is considered to be the gateway address.




IP Addressing Quiz

Welcome to your IPv4 Addressing




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