Layer 1 - Physical

Physical layer defines the cable or physical medium itself, e.g., thinnet,thicknet,unshielded
twisted pairs UTP). Converters from one media to another operate at this level.

Layer 2 - Data Link

Data Link layer defines the format of data on the network.A network data frame,aka packet,
includes checksum, source and destination address, and data.The data link layer handles the
physicaland logical connections to the packet's destination, using a network interface.A host
connected to an Ethernet would have an Ethernet interface to handle connections to the
outside world, and a loopback interface to send packets to itself.

Layer 3 - Network

IP is responsible for routing, directing datagrams from one network to another. The network
layer may have to break large datagrams, larger than MTU, into smaller packets and the host
receiving the packet will have to reassemble the fragmented datagram.
The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are
written as four dot-separated decimal numbers between 0 and 255, e.g.,129.79.16.40.The
leading 1-3 bytes of the IP identify the network and the remaining bytes identify the host
on that network.

Layer 4 - Transport

Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces
desired transmission control.Two transport protocols, Transmission Control Protocol(TCP)
and User Datagram Protocol (UDP) sits at the transport layer.
Reliability and speed are the primary difference between these two protocols.TCP establishes
connections between two hosts on the network through "sockets",which are determined
by the IP address,and port number. TCP keeps track of the packet delivery order and
the packets that must be resent. UDP on the other hand provides a low overhead transmission
service, but with less error checking.

Layer 5 - Session

The session protocol defines the format of the data sent over the connections.The NFS uses the
Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions use TCP whereas NFS and broadcast use UDP.


Layer 6 - Presentation

External Data Representation (XDR) sits at the presentation level.It converts local representation
of data to its canonical form and vice versa.The canonical uses a standard byte ordering and
structure packing convention, independent of the host.

Layer 7 - Application

Provides network services to the end-users. Mail,ftp,telnet,DNS, NIS,NFS are examples of
network applications.

Understanding IP Addressing