Default Mask

Default Mask

When a router receive a packet with a destination address, it needs to route the packet. A router outside outside the organization route the packet based on the network address and a router inside the organization route the packet based on the subnetwork address.

  • Network Address
  • Sub Network Address
  • Default Mask
  • Subnet Mask

The router outside the organization has a routing table with one coulmn based on the network address. The  router inside the organization has a routing table based on the subnetwork address.


 Class Default Mask



Functions of Data Link Layer

Functions of Data Link Layer

   It accepts a data unit (segment ) from the network layer and adds meaningful bit to the beginning  (header ) and end (trailer ).

   The header contains physical address of the source and destination.

   Trailer contains extra bits for error controlling.

   The data link layer divide the stream of bits ( segments ) received from network layer into manageable data units called frames.

   Then the header and trailer are added with header contains the physical address of sender / receiver and the trailer contains error control information.

    Data link layer controls link to link delivery of packets.

    Example for devices working in this layer are switch , bridge. 

Network layer in ISO OSI Reference Model

Functions of Network layer in ISO OSI Reference Model

                  Network layer is concerned with controlling the operations of the network.

                  Network Layer is responsible for the source to destination delivery of individual packets.

                  Network Layer provides functions for routing and multiplexing.

                  Network Layer is also responsible for congession control.

                  Network Layer receives data unit (segment) from transport layer and it adds the header information to the segment .

                 The header information   includes logical address ( ip address )     information.

                 The main protocol  in network layer is IP ( Internet Protocol ) .

                 Other protocols in network layer are IGMP , ICMP , ARP , RARP.

                 The device working in network  layer is router.

                 IP ( Internet Protocol ) is the carrier of data and it is a connectionless unreliable protocol. 

Network Topologies

Network Topologies

Network Topology Refers to the way a network is laid out either physically or logically.Network Topology  can be viewed as a geometric representation of all the links in a network.
  • Mesh Topology
  • Star Topology
  • Tree Topology
  • Bus Topology
  • Ring Topology
  • Hybrid Topology

  • In Mesh topology every node has a dedicated point to point link to every other node.
  • In star topology each device has a dedicated point-to-point link only to a central controller, usually  called a hub.The devices are not directly linked together.
  • In Tree topology nodes in a tree  are linked to a central hub.Not every device connect directly to the central hub.Nodes can also connect to a secondary hub that in turn connect to the central hub.
  • In Bus topology one cable acts as a backbone to link all the nodes in the network.
  • In Ring topology each node has a dedicated point-to-point link on either side of it.
  • Hybrid topology is a combination of several topologies of sub-networks linked together in the large topology. Different topologies are connected to each other via star topology.

History of Computer Network

History of Computer Network
In the 1960's Computers from different manufactures were unable to communicate  with one another. The advanced Research Project Agency(ARPA) in the Department of Defense (DOD) has taken interest in connecting computers so that the computers can communicate with one another.
In 1967 ARPA came  up with its ideas for ARPANET a small network of connected computers. It suggest that each host computer(from any manufacturer)would be attached to a specialized computer called an Interface Message Processor(IMP).The IMP's inturn connected to other IMP's also.That is the IMP's can communicate with other IMP's as well  as with its own attached hosts.
In 1969 the ARPANET has become a reality. Software called Network Control protocol(NCP) provided communication between hosts using IMP's.
In 1972 two core members of the ARPANET group  collaborated on a project called Internetting project.And in 1973 they outlined the protocols for the end-to-end delivery of packets(Transmission control protocol :TCP).
Later the authorities decided to split TCP into two protocols:Tranmission control protocol(TCP) and Internet Protocol(IP).