End-user devices that provide users with a connection to the network are also referred to as hosts.

NICs are sometimes called network adapters. Laptop or notebook computer NICs are usually the size of a PCMCIA card.

A repeater is a network device used to regenerate a signal.

Hubs concentrate connections. Active hubs concentrate hosts and also regenerate signals.

Bridges convert network data formats and perform basic data transmission management. Bridges provide connections between LANs.

Workgroup switches add more intelligence to data transfer management. Another difference between a bridge and switch is that a switch does not convert data transmission formats.

Routers have all the capabilities listed above. Routers can regenerate signals, concentrate multiple connections, convert data transmission formats, and manage data transfers. They can also connect to a WAN, which allows them to connect LANs that are separated by great distances.

• A bus topology uses a single backbone cable that is terminated at both ends. All the hosts connect directly to this backbone.
• A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
• A star topology connects all cables to a central point.
• An extended star topology links individual stars together by connecting the hubs or switches.
• A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.
• A mesh topology is implemented to provide as much protection as possible from interruption of service.

The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcast and token passing.

     1. The use of a broadcast topology indicates that each host sends its data to all other hosts on the network medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet works this way.

     2. The second logical topology is token passing. In this type of topology, an electronic token is passed sequentially to each host. When a host receives the token, that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself. Two examples of networks that use token passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of Token Ring and FDDI is Arcnet. Arcnet is token passing on a bus topology.

Protocol suites are collections of protocols that enable network communication between hosts.

A protocol is a formal description of a set of rules and conventions that govern a particular aspect of how devices on a network communicate. Protocols determine the format, timing, sequencing, and error control in data communication.

2.1.6 Local-area networks (LANs)

LANs allow businesses to locally share computer files and printers efficiently and make internal communications possible.

Some common LAN technologies include the following:

• Ethernet
• Token Ring
• FDDI

2.1.7 Wide-area networks (WANs)

WANs are designed to do the following:

• Operate over a large and geographically separated area
• Allow users to have real-time communication capabilities with other users
• Provide full-time remote resources connected to local services
• Provide e-mail, Internet, file transfer, and e-commerce services

Some common WAN technologies include the following:

• Modems
• Integrated Services Digital Network (ISDN)
• Digital subscriber line (DSL)
• Frame Relay
• T1, E1, T3, and E3
• Synchronous Optical Network (SONET)

Wireless bridge technologies that send signals across public areas can also be used to create a MAN.

A MAN usually consists of two or more LANs in a common geographic area.

2.1.9 Storage-area networks (SANs)

A storage-area network (SAN) is a dedicated, high-performance network used to move data between servers and storage resources. Because it is a separate, dedicated network, it avoids any traffic conflict between clients and servers.

SAN technology allows high-speed server-to-storage, storage-to-storage, or server-to-server connectivity.

SANs offer the following features:

• Performance – SANs allow concurrent access of disk or tape arrays by two or more servers at high speeds. This provides enhanced system performance.
• Availability – SANs have built-in disaster tolerance. Data can be duplicated on a SAN up to 10 km (6.2 miles) away.
• Scalability – A SAN can use a variety of technologies. This allows easy relocation of backup data, operations, file migration, and data replication between systems.

2.1.10 Virtual private network (VPN)

A vitual private network (VPN) is a private network that is constructed within a public network infrastructure such as the global Internet.

Using VPN, a telecommuter can remotely access the network of the company headquarters. Through the Internet, a secure tunnel can be built between the PC of the telecommuter and a VPN router at the company headquarters.

2.1.11 Benefits of VPNs

A VPN is a service that offers secure, reliable connectivity over a shared public network infrastructure such as the Internet. 

VPNs maintain the same security and management policies as a private network.

The following are the three main types of VPNs:

• Access VPNs provide remote access for mobile and small office, home office (SOHO) users to an Intranet or Extranet over a shared infrastructure. Access VPNs use analog, dialup, ISDN, DSL, mobile IP, and cable technologies to securely connect mobile users, telecommuters, and branch offices.
• Intranet VPNs use dedicated connections to link regional and remote offices to an internal network over a shared infrastructure. Intranet VPNs differ from Extranet VPNs in that they allow access only to the employees of the enterprise.
• Extranet VPNs use dedicated connections to link business partners to an internal network over a shared infrastructure. Extranet VPNs differ from Intranet VPNs in that they allow access to users outside the enterprise.

2.1.12 Intranets and extranets

Intranet:

One common configuration of a LAN is an intranet.

Intranet Web servers differ from public Web servers in that the public must have the proper permissions and passwords to access the intranet of an organization.

Intranets are designed to permit users who have access privileges to the internal LAN of the organization.

Extranet:

Extranets refer to applications and services that are Intranet based, and use extended, secure access to external users. This access is usually accomplished through passwords, user IDs, and other application-level security.

An extranet is the extension of two or more intranet strategies with a secure interaction between participant enterprises and their respective intranets.

The OSI reference model is a framework that is used to understand how information travels throughout a network.

Dividing the network into seven layers provides the following advantages:

• It breaks network communication into smaller, more manageable parts.
• It standardizes network components to allow multiple vendor development and support.
• It allows different types of network hardware and software to communicate with each other.
• It prevents changes in one layer from affecting other layers.
• It divides network communication into smaller parts to make learning it easier to understand.

2.3.5 Peer-to-peer communications

Each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is referred to as peer-to-peer.

During this process, the protocols of each layer exchange information, called protocol data units (PDUs).

2.3.6 TCP/IP model

The TCP/IP model has the following four layers:

• Application layer
• Transport layer
• Internet layer
• Network access layer

Some of the most commonly used application layer protocols include the following:

• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (SMTP)
• Domain Name System (DNS)
• Trivial File Transfer Protocol (TFTP)

The common transport layer protocols include:

• Transport Control Protocol (TCP)
• User Datagram Protocol (UDP)

The primary protocol of the Internet layer is:

• Internet Protocol (IP)

The network access layer refers to any particular technology used on a specific network.

2.3.7 Detailed encapsulation process

If one computer (host A) wants to send data to another computer (host B), the data must first be packaged through a process called encapsulation.

Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information.