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# System prepended metadata

title: 'CCNA: Introduction to Networks'

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# 1 | Networking Today

## 1.2 Network Components

**Hosts**: Also known as end devices, sometimes as clients. Devices on a network that are assigned a number for communication purposes, known as an Internet Protocol (IP) address. 

**Servers**: Computers with software that allows them to provide information to other end devices. Each services requires separate server software, and computers with server software can provide services simultaneously to many different clients. 

- Examples of server software includes: Email, Web, File

**Peer-to-Peer**: Computers can also function as both a server and a client, creating a peer-to-peer client. 

| Advantages                                      | Disadvantages                    |
| ----------------------------------------------- | -------------------------------- |
| Easy to set up                                  | No centralized administration    |
| Less complex                                    | Not as secure                    |
| Lower cost, no dedicated devices/servers needed | Not scalable                     |
| Can be used for simple tasks                    | Can slow performances of devices |

**End Devices**: Each end device has an address; using the destination end device to speicfy where to deliver the message. 

**Intermediary Devices**: Connect end devices to the network, connecting multiple networks to form an internetwork. Provides connectivity and data flow across the network. 

> Intermediary Devices use the destination end device address to determine paths that messages should take. 

**Network Media**: Provides the channel over which the message travels from source to destination; there are 3 modes - copper (electrical impulses), fiber-optic (light pulses), and wireless trandmission (modulation of specific frequencies of electromagnetic waves)

## 1.3 Network Representations and Topologies

- **Network Interface Card**: Physically connects end device to network
- **Physical Port**: Connector or outlet on a networking device where media connects to an end device or another networking device
- **Interface**: Specialized ports on a networking device that connect to individual networks; ports on a router are known as network interfaces

> Port and interface are often used interchangably

There are two kinds of topology diagrams: Physical, and Logical

- Physical: physical location of devices, cable installation; rooms that devices are located in.
- Logical: ports, devices, addressing scheme of the network; see what end devices are connected and what media is being used. 

## 1.4 Networks of Many Sizes

- Small Home Network: couple of computers connected to each other & internet
- Small Office and Home Office Network (SOHO): computers in home/remote office to connect to a corporate network/centralized access/shared resources
- Medium / Large networks: Corporations, schools, many locations with hundreds or thousands of interconnected hosts
- World Wide Networks: Hundreds of millions of computers world-wide

There are two types of network infrastructures - Local Area Networks (LANs) and Wide Area Networks (WANs). 

- For LANs: provides access to users and end devices in small geographical area
    - Interconnect end devices in limited area, administered by single entity, high-speed bandwidth to internal devices
- For WANs: provides access to other networks over a wide geographical area, typically owned and managed by a larger corporation/telecommunications service provider
    - Connects different LANs together, administered by multiple service providers, slower speed links between LANs
    - WANs can connect through copper wires, fiber-optic, and wireless transmissions.

Intranets are private connections of LANs and WANs that belongs to an organization, designed to be accessible only by the organization's members or others with authorization. Extranets are for individuals in adjacent organizations that require access to the organization's data.

## 1.5 Internet Connections

For small businesses, there are a couple of ways to be connected: Cable, Digial Subscriber Line (DSL), Cellular, Satellite, Dial-Up

- Cable: from TV service providers, signal transmits on the same cable that delivers cable television; high bandwidth, availability, and an always-on connection to the internet
- DSL: high bandwidth, available, always-on connection; runs over telephone line. Generally small offices have asymmetric DSL where the download speed is faster than the upload speed
- Cellular: cell phone network to connect, limited by capabilities of phone and cell tower to which it is attached
- Satellite: for areas that would otherwise have no internet connectivity, needs a clear line of sight to satellite
- Dial-up: inexpensive, phone line + modem; not good for large data transfer

For large businesses, they can be connected by Dedicated Leased Lines, Metro Ethernet, Business DSL, Satellite

- Dedicated Leased Lines: Reserved circuits within the service provider's network that connect geographically separated offices; rented at a yearly or monthly rate
- Metro Ethernet: Ethernet WAN, extends LAN access technology to a WAN
- Business DSL: Symmetric DSL, both uploads and downloads at the same speed
- Satellite: connection when wired solutions are not available

Networks can be traditionally separated (cannot comunicate with each other), or converged (different types of services can be provided)

## 1.6 Network Architecture

**Fault Tolerance**: limited impact during a failure, allows for a quick recovery; networks depend on multiple paths between the source and destination. Can be done by packet switching. 

**Scalability**: ability to expand quickly to support new users and applications without degrading the performance of other users

**Quality of Service**: manage congestion and ensure reliable delivery of content; congestions occurs when more data is being sent than what the network can handle. With QoS, priority will be given to more critical services (ie. VoIP vs downloading a file) 

**Security**: security is important, network admins focus on security infrastructure and information; through securing devices and preventing unauthorized access to ensure CIA

## 1.7 Network Trends

- Bring Your Own Device (BYOD): users choose what device to access information and services needed in an organization
- Online Collaboration: Zoom, Google Meets, Google Docs; ability to work with each other virtually. Cisco Webex Teams is a service that is basically Microsoft Teams
- Video Communication: Video Conferencing
- Cloud Computing: Store files virtually, no need for server on-premises, can access services through the cloud. Requires data centers; data is often stored in distributed data centers across different locations
    - Public, Private, Hybrid, Community cloud
- Homes are using more IoT devices to make smart homes 
- Powerline Networking: use existing wiring to connect devices; devices connect to LAN where there is electricity; useful when wireless access points cannot reach all the devices in a home - though it is not a substitute for dedicated cabling in data networks
- Wireless Broadband: wireless ISPs connect subscribers using wireless technology, common in rural areas. A dish or antenna is installed in range of the WISP transmitter, connecting to the wired network in the customer's home

## 1.8

> Know your viruses, worms, trojan horses, spyware/adware, zero-days, threat actors, (D)DOS, data interception, identity theft

> Know that you can use antiviruses/antispyware, firewalls, ACLs, IPSs, and VPNs as security mechanisms

# 4 | Physical Layer

The physical layer provides the means to transport the bits that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link and encodes it as a series of signals to transmit over the local media. 

Bandwidth: The capacity at which a medium can carry data, determined by the proteries of physical media and the technologies chosen for signaling and detecting network signals.

- Latency: amount of time for data to travel from one given point to another. 
- Throughput: the measure of the transfer of bits across the media over a given period of time
- Goodput: The measure of usable data transferred over a given period of time; throughput minus traffic overhead.

Copper cabling is the most common type of cabling. Data is transmitted as electrical pulses; but is susceptible to interference from two sources - EMI/RFI and crosstalk

- EMI/RFI is countered by wrapping copper cables in metallic shielding and proper grounding connections
- Crosstalk is countered by twisting opposing circuit wire pairs together. 

UTP - most common type of networking media, four pairs of color-coded wires that have been twisted together and encased in flexible plastic that protects from minor physical damage. 

- UTP cables limit crosstalk with cancellation and varying the number of twists per wire pair
- Straight-through Ethernet cables are used to interconnect a host to a switch and a switch to a router
- Crossover Ethernet cables are used to interconnect similar devices
- Rollover cables are Cisco proprietary that connects a workstation serial port to a router console port using an adapter.

STP - better noise protection than UTP, more expensive and difficult to install; combines techniques of shielding to counter EMI/RFI, and wire twisting to counter crosstalk. STP cables are terminated with special shielded STP data connections.

Coaxial Cable - Copper conductor to transmit electronic signals, layer of flexible plastic insulation, surrounded in a woven copper braid, and then covered with a cable jacket. Coax are used for wireless installations and cable internet installations.

Fiber-Optic - Immune for EMI/RFI, connects network devices. Flexible, thin. Has single and multi-mode.

- Single-mode: small core, expensive laser technology, long-distance situations
- Multi-mode: larger core, LED emitters to send light pulses. Light enters at different angles, popular in LANs.

Multimode has greater dispersion than single.

Fiber is used in enterprise networks, Fiber-to-the-Home, Long-haul networks, and submarine cable networks

Straight-tip connectors: One of the first connector types used, locks security with a twist-on/twist-off

Subscriber Connector: Square connectors, widely adopted LAN/WAN connector that uses a push-pull mechanism; used in multimode and singlemode

Lucent Connector Simplex: Smaller version of SC

Duplex Multimode LC Connector: Similar to an LC simplex connector, but uses a duplex connector. 

![image](https://hackmd.io/_uploads/SySh6d0rWl.png)

# 6 | Data Link Layer

IEEE 802 LAN/MAN data link layer has two sublayers - Logical Link Control (LLC) and Media Access Control (MAC). The LLC communicates between the networking software at the upper layers and the device hardware at the lower layers, placing information in the frame that identifies which network layer protocol is being used for the frame. The MAC is responsible for data encapsulation and media access control, providing the data link layer addressing and is integrated with various physical layer technologies.

# 7 | Ethernet Switching

