Basic Networking

OSI Model

A conceptual model of networking based on seven “layers”:

#	Layer	Description
7	Application	Provides networking options (an interface to transmit data) to programs running on the computer.
6	Presentation	Translates the data into a standardized format (so the receiving machine can read it); it also handles encryption, compression, or other transformations of the data.
5	Session	Sets up and maintains a connection with the other computer across the network (connection is unique to this communication).
4	Transport	Decides on a transport protocol (TCP or UDP) and then divides up the data to be sent over the network (TCP: segments; UDP: datagrams).
3	Network	[router level] Locates the destination computer via logical addressing (i.e., IP Addresses).
2	Data Link	[switch level] Identifies the MAC/hardware addresses of the machines on the network (mapping each IP Address to the device's MAC/hardware address). Runs a CRC for data integrity and then presents the data in a suitable format for transmission.
1	Physical	The hardware that communicates with electrical impules (1's and 0's) or light (on and off).

The machine that receives the transmssion does so on Layer 1, then passes the data up the stack to Layer 2, etc.

#	Layer	Description
4	Application	Provides applications with standardized data exchange via protocols like HTTP, FTP, POP3, SMTP, and SNMP. The payload is the actual application data.
3	Transport	Responsible for maintaining end-to-end communications across the network via TCP and UDP protocols.
2	Internet	Deals with packets and connects independent networks to transport the packets across network boundaries via protocols like ICMP.
1	Network Access	The “physical” or “link” layer; defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. Protocol: Ethernet.

The TCP/IP model was introduced first (in 1982 by the US DoD) and is the standard upon which modern networking is based.
The OSI model was developed later as a more comprehensive guide for learning.