include source and destination routing information. The routing information in each
packet informs the network where to send the packet to reach its destination and tells
the receiving computer from where the packet originated.
The network layer is most important when the network connection passes through
one or more routers, which are hardware devices that examine each packet and, from
their source and destination addresses, send the packets to their proper destination.
Over a complex network, such as the Internet, a packet might go through ten or more
routers before it reaches its destination. On a LAN, a packet might not go through any
routers to get to its destination, or it might go through one or more.
Note that breaking the network layer (also known as the packet layer) into a separate
layer from the physical and data-link layers means the protocols defined in this layer
can be carried over any variations of the lower layers. So, to put this into real-world
terms, an IP packet can be sent over an Ethernet network, a Token Ring network, or
even a serial cable that connects two computers. The same holds true for an IPX packet:
If both computers can handle IPX, and they share the lower-level layers (whatever they
might be) in common, then the network connection can be made.
The transport layer, layer 4, manages the flow of information from one network node
to another. It ensures that the packets are decoded in the proper sequence and that all
packets are received. It also identifies each computer or node on a network uniquely.
The various networking systems (such as Microsoft's, or Novell's) implement the
transport layer differently. In fact, the transport layer is the first layer where differences
between network operating systems occur.
Examples of transport layer protocols include Transmission Control Protocol (TCP)
and Sequenced Packet Exchange (SPX), which are used in concert with IP and IPX,
The session layer, layer 5, defines the connection from a user computer to a network
server, or from a peer computer on a network to another peer computer. These virtual
connections are referred to as sessions. They include negotiation between the client and
host (or peer and peer) on matters of flow control, transaction processing, transfer of
user information, and authentication to the network. They are called sessions because
they set up connections that persist for some period of time.
The presentation layer, layer 6, takes the data supplied by the lower-level layers and
transforms it so it can be presented to the system (as opposed to presenting the data to
the user, which is handled outside the OSI model). The functions that take place at the
presentation layer can include data compression and decompression, as well as data
encryption and decryption.