CCNA flash cards

A network is a collection of devices and end systems.Networks consist of computers, servers, and network devices, such as switches and routers, that can communicate with each other.

• - Personal computers (PCs): Send and receive data and are the endpoints of the network.
• - Interconnections: The components that provide a means for data to travel across the network. This includes network interface cards (NIC), network media, and connectors.
• - Switches: Provide network access for the PCs.
• - Routers: Interconnect networks.

• The four major resources that are shared on a computer network are as follows:
• - Data and applications: Consist of computer data and network-aware applications such as e-mail
• - Resources: Include input and output devices such as cameras and printers
• - Network storage: Consists of directly attached storage devices (physical storage that is directly attached to a computer and a shared server), network attached storage, and storage area networks
• - Backup devices: Can back up files and data from multiple computers

• The most common network user applications on today's networks are as follows:
• - E-mail
• - Web browsers
• - Instant messaging
• - Collaboration
• - Databases

• Three categories of network applications are as follows:
• - Batch applications: Examples are FTP and TFTP. They are started by a human and complete with no other interaction.
• - Interactive applications: Include database updates and queries. A person requests data from the server and waits for a reply. Response time depends more on the server than the network.
• - Real-time applications: Include VoIP and video. Network bandwidth is critical because these applications are time critical. Quality of service (QoS) and sufficient network bandwidth are mandatory for these applications.

• Two types of network-monitoring software are as follows:
• - Protocol analyzers: Capture network packets between computers and decode the packets so that one can view what is occurring during transmission
• - Sniffers: Work like a wiretap and allow you to not only observe communication between computers but also view what is being transmitted

Speed refers to how fast data is transmitted over the network.

Cost refers to the general cost of network components, installation, and maintenance.

Security refers to how secure the network and network data are.

Availability is the measure of the likelihood that the network will be available for use when required.Network availability can be calculated using the following formula, which calculates the minutes of downtime compared to the number of minutes in a year:([525,600 [ms] Minutes downtime]/[525,600]) * 100

99.994%.([525,600 [ms] 30]/[525,600]) * 100 = 99.994%

Scalability refers to how well the network can accommodate more users and more data.

Reliability refers to the dependability of the devices that make up the network (switches, routers, PCs, servers, and so on).

Physical topology defines the physical components of the network: cables, network devices, and computers.Logical topology defines the data path of the network.

Three common methods (technologies) used to connect to the Internet are digital subscriber line (DSL), cable, and serial.

• The five classes of network attacks are as follows:
• - Passive: Include capturing and monitoring unprotected communication and capturing passwords. The attacker gains access to information or data without the consent or knowledge of users.
• - Active: Actively try to break or bypass security devices, introduce malicious code, and steal and modify data.
• - Close-in: Occur when an individual attains close physical proximity to networks or facilities with the intent of gathering or changing data.
• - Insider: Occur from authorized users inside a network. They can be either malicious or nonmalicious.
• - Distribution: Focus on the malicious changes to hardware or software at the factory or during distribution to introduce the malicious code to unsuspecting users.

This is a passive attack.

This is a close-in and passive attack.It is passive because the hacker is monitoring and gathering data. It is also close-in because the hacker had to gain close proximity to the company's wireless network to capture the wireless data.

• The four steps of the Cisco security wheel are as follows:
• - Step 1: Secure
• - Step 2: Monitor
• - Step 3: Test
• - Step 4: Improve

Reconnaissance attacks are attacks that gather information about the target.Types of attacks include sniffers, ping sweeps, port scans, and Internet Domain Name System (DNS) queries.

Access attacks exploit known web services, databases, operating systems, and authentication services.

• Five types of access attacks are as follows:
• - Password attacks
• - Trust exploitation
• - Port redirection
• - Man-in-the-middle attacks
• - Buffer overflow

• Some ways to mitigate password attacks are as follows:
• - Disable accounts after a specific number of unsuccessful login attempts.
• - Do not use plain-text passwords.
• - Do not allow users to share the same password on different systems.
• - Use strong passwords.

Telnet sends all data in clear text, including passwords. SSH encrypts all data, ensuring that passwords and session data are secured.

• Six reasons that the OSI reference model was created are as follows:
• - To ensure that different vendors' products can work together
• - To create standards to enable ease of interoperability by defining standards for the operations at each level
• - To clarify general functions of internetworking
• - To divide the complexity of networking into smaller, more manageable sublayers
• - To simplify troubleshooting
• - To enable developers to modify or improve components at one layer without having to rewrite an entire protocol stack

• The seven layers of the OSI reference model are as follows:
• - Layer 7: Application layer
• - Layer 6: Presentation layer
• - Layer 5: Session layer
• - Layer 4: Transport layer
• - Layer 3: Network layer
• - Layer 2: Data link layer
• - Layer 1: Physical layer

The physical layer defines the physical medium. It defines the media type, the connector type, and the signaling type (baseband versus broadband). This includes voltage levels, physical data rates, and maximum cable lengths. The physical layer is responsible for converting frames into electronic bits of data, which are then sent or received across the physical medium. Twisted-pair, coaxial, and fiber-optic cable operate at this level. Other implementations at this layer are repeaters/hubs.

The data link layer defines how data is formatted from transmission and how access to the physical media is controlled. This layer also typically includes error correction to ensure reliable delivery of data.The data link layer translates messages from the network layer into bits for the physical layer, and it enables the network layer to control the interconnection of data circuits within the physical layer. Its specifications define different network and protocol characteristics, including physical addressing, error notification, network topology, and sequencing of frames.Data-link protocols provide the delivery across individual links and are concerned with the different media types, such as 802.2 and 802.3. The data link layer is responsible for putting 1s and 0s into a logical group. These 1s and 0s are then put on the physical wire. Some examples of data link layer implementations are IEEE 802.2/802.3, IEEE 802.5/802.2, packet trailer (for Ethernet, frame check sequence [FCS], or cyclic redundancy check [CRC]), Fiber Distributed Data Interface (FDDI), High-Level Data Link Control (HDLC), and Frame Relay.

• The IEEE defines the following two sublayers of the data link layer:
• - The Logical Link Control (LLC) sublayer
• - The Media Access Control (MAC) sublayer
• These two sublayers provide physical media independence.

The Logical Link Control (802.2) sublayer is responsible for identifying different network layer protocols and then encapsulating them to be transferred across the network.Two types of LLC frames exist: Service access point (SAP) and Subnetwork Access Protocol (SNAP). An LLC header tells the data link layer what to do with a packet after it is received.

• The MAC sublayer specifies how data is placed and transported over the physical wire. It controls access to the physical medium.
• The LLC sublayer communicates with the network layer, but the MAC sublayer communicates downward directly to the physical layer. Physical addressing (MAC addresses), network topologies, error notification, and delivery of frames are defined at the MAC sublayer.

Bridges and switches operate at the data link layer.Both devices make decisions about what traffic to forward or drop (filter) by MAC addresses, and logical network address are not used at this layer. Data link layer devices assume a flat address space.

• The network layer provides internetwork routing and logical network addresses. It defines how to transport traffic between devices that are not locally attached.
• The network layer also supports connection-oriented and connectionless service from higher-layer protocols.Routers operate at the network layer. IP, Internetwork Packet Exchange (IPX), AppleTalk, and Datagram Delivery Protocol (DDP) are examples of network layer implementations.

• Network layer addresses are logical.
• These addresses are logical addresses that are specific to the network layer protocol being run on the network. Each network layer protocol has a different addressing scheme. They are usually hierarchical and define networks first and then hosts or devices on that network. An example of a network address is an IP address, which is a 32-bit address often expressed in decimal format. An example of an IP address in decimal format is 192.168.0.1.

• The transport layer segments and reassembles data from upper-layer applications into data streams. It provides reliable data transmission to upper layers.
• End-to-end communications, flow control, multiplexing, error detection and correction, and virtual circuit management are typical transport layer functions. Some examples include TCP, User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX).

• Flow control is the method of controlling the rate at which a computer sends data, thus preventing network congestion.The three methods of implementing flow control are as follows:
• - Buffering
• - Source-quench messages (congestion avoidance)
• - Windowing

• The session layer is responsible for creating, managing, and ending communication sessions between presentation layer entities.
• These sessions consist of service requests and responses that develop between applications located on different network devices. Some examples include Structured Query Language (SQL), remote-procedure call (RPC), X Window System, Zone Information Protocol (ZIP), NetBIOS names, and AppleTalk ASP.

• Also known as the translator, the presentation layer provides coding and conversion functions to application layer data. This guarantees that the application layer on one system can read data transferred from the application layer of a different system. Some examples of the presentation layer are as follows:
• - Compression, decompression, and encryption
• - JPEG, TIFF, GIF, PICT, QuickTime, MPEG, EBCDIC, and ASCII file types

• The application layer is the layer that is closest to the user. This means that this layer interacts directly with the software application. The application layer's main function is to identify and establish communication partners, determine resource availability, and synchronize communication. Some examples include the following:
• - TCP/IP applications such as Telnet, FTP, Simple Mail Transfer Protocol (SMTP), and HTTP
• - OSI applications such as Virtual Terminal Protocol; File Transfer, Access, and Management (FTAM); and Common Management Information Protocol (CMIP)

Each layer of the OSI model can communicate only with the layer above it, below it, and parallel to it (a peer layer).For example, the presentation layer can communicate with only the application layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using service access points (SAP) and protocol data units (PDU). The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer. PDUs control information that is added to the user data at each layer of the model. This information resides in fields called headers (the front of the data field) and trailers (the end of the data field).

• Encapsulation wraps data with the necessary protocol information before network transmission.A PDU can include different information as it goes up or down the OSI model. It is given a different name according to the information it is carrying (the layer where it is located). When the transport layer receives upper-layer data, it adds a TCP header to the data; this is called a segment. The segment is then passed to the network layer, and an IP header is added; thus, the data becomes a packet. The packet is passed to the data link layer, thus becoming a frame. This frame is then converted into bits and is passed across the network medium. This is data encapsulation. For the ICND test, you should know the following:
• - Application layer: Data
• - Transport layer: Segment
• - Network layer: Packet
• - Data link layer: Frame
• - Physical layer: Bits

• The four layers of the TCP/IP stack are as follows:
• - Application
• - Transport
• - Internet
• - Network Access

Physical data rates, connectors, and MAC addresses are located on the network access layer.

• Some protocols that operate at the TCP/IP Internet layer are as follows:
• - IP
• - ICMP (Internet Control Message Protocol)
• - ARP (Address Resolution Protocol)
• - RARP (Reverse Address Resolution Protocol)