2.1 Networking Overview

Networking Facts

A network is a group of computers that can share information through interconnections.

Networking Components

A network is made up of the following components:

• Computers - these are often called nodes or hosts.
• Transmission media - a path for electrical signals between devices.
• Network interfaces - devices that send and receive electrical signals.
• Protocols - rules or standards that describe how hosts communicate and exchange data.

Network Benefits

Despite the costs of implementation and maintenance, networks save organizations money by allowing them to:

• Consolidate (centralize) data storage.
• Share peripheral devices (such as printers).
• Increase internal and external communications.
• Increase productivity and collaboration.

Network Classification

Host role

Peer-to-peer

• In a peer-to-peer network, each host can provide network resources to other hosts or use resources located on other hosts. Each host providing resources controls access to the resources.

Advantages of peer-to-peer networks include:

• Easy implementation
• Inexpensive

Disadvantages of peer-to-peer networks include:

• Difficult to expand (not scalable)
• Difficult to support
• Lack centralized control
• No centralized storage

Client-server In a client-server network, hosts have specific roles. For example, some hosts are assigned server roles that allow them to provide network resources to other hosts. Other hosts are assigned client roles that allow them to consume network resources.

Advantages of client-server networks include the following:

• Easy to expand (scalable)
• Easy to support
• Centralized services
• Easy to back up

Disadvantages of client-server networks include the following:

• Expensive server operating systems
• Extensive advanced planning required

Geography

Personal area network (PAN)

• A personal area network is a small network used for communicating between personal devices. For example, a PAN may include a notebook computer, a wireless headset, a wireless printer, and a smart phone.
• A PAN is limited in range to only a few feet. A PAN is typically created using Bluetooth wireless technologies.

Local area network (LAN)

• A local area network is a network in a small geographic area, like an office. A LAN typically uses wires to connect systems together.

Wireless local area network (WLAN)

• A wireless LAN covers an area that is roughly the same size as a standard LAN. However, it uses radio signals instead of wires to connect systems together.

Wide area network (WAN)

• A wide area network is a group of LANs that are geographically isolated and connected to form a large internetwork.

Metropolitan area network (MAN)

• A metropolitan area network is a network that covers an area as small as a few city blocks to as large as an entire metropolitan city. MANs are typically owned and managed by a city as a public utility.
• Be aware that many IT professionals do not differentiate between a wide area network and a MAN as they use the same network technologies.

Campus area network (CAN)

• A campus area network is a computer network made up of an interconnection of local area networks within a limited geographical area, such as a university's campus.

Most of the networking components, such as the transmission media, switches, and routers are owned by the campus tenant.

Software-defined wide area network (SD-WAN)

SD-WAN A software-defined wide area network is a virtual WAN network that uses software to control connectivity, management and services between data centers and remote branches as well as the cloud.

It allows any combination of transport services such as Long Term Evolution (LTE), Multiprotocol Label Switching (MPLS), and broadband internet services, to securely connect users to applications.

SD-WAN is based on the same methodology as software-defined networking (SDN) which separates the control plane from the data plane to make networking more intelligent.

Management

Network The term network often describes a computer system controlled by a single organization. This could be a local area network at a single location or a wide area network used by a single business or organization.

If two companies connect their internal networks to share data, you could call it one network. However, it is two networks because each network is managed by a different company.

Subnet A subnet is a portion of a network with a common network address.

• All devices on the subnet share the same network address, but they have unique host addresses.
• Each subnet in a larger network has a unique subnet address.
• Devices connected through hubs or switches are on the same subnet. Routers are used to connect multiple subnets.

Internetwork An internetwork has geographically dispersed WAN connections that connect multiple LANs.

Connecting two networks under different management is a form of internetworking because data must travel between two networks.

Participation

Internet The internet is a large world-wide public network. The internet is public because virtually anyone can connect to it.

• Users and organizations connect to the internet through an internet service provider (ISP).
• The internet uses a set of communication protocols (TCP/IP) for providing services.
• Individuals and organizations can make services (such as a website) available to other users on the internet.

Intranet An intranet is a private network that uses internet technologies. Services on an intranet are available only to hosts that are connected to the private network.

For example, your company might have a website that only employees can access.

Extranet An extranet is a private network that uses internet technologies, but its resources are made available only to external, trusted users.

For example, you might create a website on a private network that only users from a partner company can access.

Dynamic Multipoint VPNs

Dynamic Multipoint VPN (DMVPN) is a Cisco software solution for building multiple VPNs in an easy, dynamic, and scalable manner. You use a DMVPN when you need to add more sites, but other VPN types are not sufficient.

• DMVPN uses a hub-and-spoke configuration to establish a full mesh topology. This simplifies the VPN tunnel configuration and provides a flexible option to connect to the central site.
• Each site uses Multipoint Generic Routing Encapsulation (mGRE). The mGRE tunnel interface allows a single GRE interface to dynamically support multiple IPsec tunnels.
• Spoke sites can also obtain information about each other and alternatively build direct tunnels between themselves (spoke-to-spoke tunnels).

Network Topology

Physical Topology

The physical topology describes the way the network is wired. The following table describes several common physical topologies:

Topology	Description
Bus	A bus topology consists of a trunk cable with nodes either inserted directly into the trunk or tapped into the trunk using offshoot cables called drop cables. When using a bus topology:

Signals travel from one node to all other nodes. A device called a terminator is placed at both ends of the trunk cable. Terminators absorb signals and prevent them from reflecting repeatedly back and forth on the cable. It can be difficult to isolate cabling problems.

A broken cable anywhere on the bus breaks the termination and prevents communications between all devices on the network. | | Ring | A ring topology connects neighboring nodes until they form a ring. Signals travel in one direction around the ring. Each device on the network acts as a repeater to send the signal to the next device. With a ring:

Installation requires careful planning to create a continuous ring. Problem isolation can require going to several physical locations along the ring. A node malfunction or cable break can prevent signals from reaching nodes beyond the malfunction. | | Star | A star topology uses a hub or switch to connect all network connections to a single physical location. Star is the most commonly used type of topology for a LAN. With a star:

All network connections are located in a single place. This makes it easy to troubleshoot and reconfigure. You can easily add or remove nodes from the network. Cabling problems usually affect only one node. | | Mesh | A mesh topology exists when there are multiple paths between any two nodes on a network. Mesh topologies are created using point-to-point connections. This increases the network's fault tolerance because alternate paths can be used when one path fails. Two variations of mesh topologies exist:

Partial mesh—Some redundant paths exist. Full mesh—Every node has a point-to-point connection with every other node.

Full mesh topologies are usually impractical because:

The number of connections increases dramatically with every new node added to the network of a standard LAN. They require a separate network interface and cable for each host on the network.

A full mesh topology is commonly used to:

Interconnect routers. This provides alternate paths should one path go down or become overloaded. Create redundant paths between access points in a wireless network. This provides alternate paths back to the wireless controller should one access point go down or become overloaded. With this topology, every access point can communicate directly with any other access point on the wireless network. | | Hybrid | A hybrid topology combines two or more topologies together to help with scalability. These combinations often include bus topology, star topology, ring topology, mesh topology (full or partial), or wireless topology.

One of the more popular hybrid topologies is the star-bus. It can also be called a tree topology. In this combination, the bus becomes the backbone of the network and each switch connects a traditional star to the bus. Another very common hybrid topology is the star-ring. In this combination, two or more star topologies are connected together through a ring topology. |

Logical Topology

The logical topology describes the way messages are sent. You should be able to identify the physical topology by looking at the way devices are connected. However, it is not as easy to identify the logical topology. As the following table shows, there is often more than one way for messages to travel in a given physical topology.

Logical Topology	Physical Topology	Description
Bus	Bus	Messages are sent to all devices connected to the bus.
Star
Ring	Ring	Messages are sent from device to device in a predetermined order until they reach the destination device.
Star
Star	Star	Messages are sent directly to (and only to) the destination device.
Mesh	Mesh	Messages are sent from one device to the next until they reach the destination device.