The OSI model is a layered mechanism that describes how information from an application on a network device moves from the source to the destination using a physical medium, and then interacts with the software application on that specific network device. The OSI model is comprised of the following seven layers:
|Layer 7: Application||Provides services to the lower layers. Enables program-to-program communication and determines whether sufficient resources exist for communication. Examples are e-mail gateways (SMTP), TFTP, FTP, and SNMP (Simple Network Management Protocol).|
|Layer 6: Presentation||Presents information to the Application layer. Compression, data conversion, encryption, and standard formatting occur here. Contains data formats such as JPEG, MPEG, MIDI, and TIFF.|
|Layer 5: Session||Establishes and maintains communication sessions between applications (dialogue control). Sessions can be simplex (one direction only), half-duplex (one direction at a time), or full duplex (both ways simultaneously). Session Layer keeps different applications data separate from other applications. Protocols include NFS, SQL, X Window, RPC, ASP, and NetBios Names.|
|Layer 4 : Transport||Responsible for end-to-end integrity of data transmissions, and establishes a logical connection between sending and receiving hosts via virtual circuits. Windowing works at this level to control how much information is transferred before acknowledgement is required. Data is segmented and reassembled at this layer. Port numbers are used to keep track of different conversations crossing the network at the same time. Supports TCP, UDP, SPX, NBP. Segmentation and error correction works here, but not detection.|
|Layer 3: Network||Routes data from one node to another and determines the best path to take. Routers operate at this level. Network addresses are used here for routing (packets). Routing tables, subnetting, and control of network congestion occur here. Routing protocols, regardless of which protocol they run over, reside here. Examples include RIP, IP, IPX, ARP, IGRP, and AppleTalk.|
|Layer 2: Data Link||Sometimes referred to as the LAN layer. Responsible for the physical transmission of data from one node to another. Error detection occurs here. Packets are translated into frames here and hardware address is added. Bridges and switches operate at this layer. Contains the LLC and MAC Sublayers.|
|Layer 1: Physical||Puts data onto the wire and includes Physical Layer specifications, such as connectors, voltage, physical data rates, and DTE/DCE interfaces. Some common implementations include Ethernet/IEEE 802.3, FastEthernet, and Token Ring/IEEE 802.5.|
Protocols are sets of rules. Network devices need to agree on a set of rules in order to communicate, and they must use the same protocol to understand each other. A wide variety of network protocols exists at different OSI layers. For example, at the lower OSI layers, LAN and WAN protocols are used. Going up the reference model, routed and routing protocols are found at Layer 3. Each layer and its associated protocols are described below.
A Protocol Data Unit (PDU) is a grouping of data used to exchange information at a particular OSI layer. The Layer 1 to Layer 4 PDU types, signifying the group of data and the specific headers and trailers, are summarized below:
Networks can be classified into the following categories based on the devices and areas they interconnect:
- A Local Area Network (LAN) is a localized computerized network used to communicate between host systems, generally for sharing information (e.g., documents, audio files, video files, e-mail, or chat messages) and using a wide variety of productivity tools.
- A Wide Area Network (WAN) is usually located over a broad geographical area and belongs to an Internet Service Provider that might charge a fee for using its WAN services.
The TCP/IP protocol suite is a modern adaptation of the OSI model and contains the following five layers:
- Data Link
Layer 2 addresses are also called Media Access Control (MAC) addresses, physical addresses, or burned-in addresses (BIA). These are assigned to network cards or device interfaces when they are manufactured.
Although each network interface has a unique MAC address, this does not specify the location of a specific device or to what network it is attached, meaning a router cannot determine the best path to that device. In order to solve this problem, Layer 3 addressing is used.
IPv4 addresses are 32-bit numbers that are represented as strings of 0s and 1s. IPv6 addresses are 128 bits long, which means a larger pool of IPv6 addresses is available. The notion of IPv6 addresses is also different: while an IPv4 address can be written in decimal format, an IPv6 address is notated in a hexadecimal format (i.e., 16 bits separated by colons), for example:
The Spanning-Tree Protocol (STP), defined by IEEE 802.1D, is a loop-prevention protocol that allows switches to communicate with each other in order to discover physical loops in a network. Switches go through the following three steps for their STP convergence:
- Elect one Root Bridge
- Elect one Root Port per non-Root Bridge
- Elect one Designated Port per segment
All STP decisions are based on a predetermined sequence, as follows:
- Lowest Root BID
- Lowest Path Cost to Root Bridge
- Lowest Sender BID
- Lowest Port ID
Virtual LANs (VLANs) define broadcast domains in a Layer 2 network. They represent an administratively defined subnet of switch ports that are in the same broadcast domain, the area in which a broadcast frame propagates through a network.
VLANs represent a group of devices that participate in the same Layer 2 domain and can communicate without needing to pass through a router, meaning they share the same broadcast domain. Best design practices suggest a one-to-one relationship between VLANs and IP subnets. Devices in a single VLAN are typically also in the same IP subnet.
IP routing is the process of forwarding a packet based on the destination IP address. Routers keep the best path to destinations learned via direct connections, static routing, or dynamic routing in internal data structures called routing tables. A routing table contains a list of networks the router has learned about and information about how to reach them.
The most important information a routing table contains includes the following items:
- How the route was learned (i.e., static, dynamic, or directly connected)
- The address of the neighbor router from which the network was learned
- The interface through which the network can be reached
- The route metric, which is a measurement that give routers information about how far or how preferred a network is (the exact meaning of the metric value depends on the routing protocol used)