IP addressing is the role of the layer-3 ‘Network Layer’ in OSI Model or Layer-2 ‘Internet layer’ in TCP/IP model.
The IP header is used to support two key operations, routing and fragmentation.
The IPv4 addresses are classified into following five categories:
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.
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 :
Application of the subnet mask to a given IP address enables:
Such information provides the basis for effective network address planning.
network address = (IP Address) . (Subnet Mask)
Let number of bits in Host address = n
Total number of hosts = 2n
Number of usage address = 2n – 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
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
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.
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.
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.