CCNA

• A network allows nodes to share resources
• A node can be a router, switch, firewall, server, or client
• A client is a device that accesses a service made available by a server
• A server is a device that provides functions or services for clients
• Switches forward traffic within a LAN (Local Area Network)
• Switches do not provide connectivity between LANs or over the Internet
• Routers connect LANs and provide connectivity between them and over the Internet
• Routers characteristics:

  • Routers are used to provide connectivity BETWEEN LANs
  • Routers are used to send data over the Internet
  • Routers have fewer network interfaces than switches
• Firewalls characteristics:

  • Firewalls control network traffic based on configured rules
  • Firewalls can be placed 'inside' or 'outside' the network
  • Next-generation firewalls include more modern and advanced filtering capabilities
• Types of firewalls:

  • Network firewalls are hardware devices that filter traffic between networks
  • Host-based firewalls are software applications that filter traffic entering and exiting a host machine, like a PC
• Firewall functions:

  • Firewalls must be configured with security rules to determine which network traffic should be allowed and which should be denied
  • Firewalls protect end hosts inside the network, like PCs and Servers
• The physical layer of the OSI model defines physical characteristics of the medium used to transfer data between devices
• Examples of physical characteristics include voltage levels, maximum transmission distances, physical connectors, and cable specifications
• Digital bits are converted into electrical signals for wired connections and radio signals for wireless connections
• The data link layer (Layer 2) provides node-to-node connectivity and data transfer
• Layer 2 defines how data is formatted for transmission over a physical medium
• Layer 2 addresses are separate from Layer 3 addresses (IP addresses)
• Switches operate at Layer 2
• A LAN (Local Area Network) is a network contained within a relatively small area, like an office floor or a home network
• Routers are used to connect separate LANs
• Switches do not separate LANs, but adding more switches can expand an existing LAN
• An Ethernet frame encapsulates a packet with a header and trailer
• The Ethernet header includes fields for preamble, start frame delimiter, destination, source, and type/length
• The Ethernet trailer includes the FCS (frame check sequence) field
• The FCS is used to detect corrupted data by running a CRC algorithm over the received data
• Fields in the Ethernet header:

  • Preamble: 7 bytes long for synchronization
  • Start Frame Delimiter (SFD): 1 byte to indicate the start of the frame
  • Destination: Layer 2 address where the frame is being sent
  • Source: Layer 2 address of the device sending the frame
  • Type/Length: 2 bytes to indicate the Layer 3 protocol or length of the encapsulated packet
• MAC addresses are 6-byte (48-bit) addresses assigned to physical devices
• MAC addresses are separate from logical addresses like IP addresses
• The Type/Length field in the Ethernet header can indicate either the type or length of the encapsulated packet
• A value of 1500 or less in the Type/Length field indicates the length of the encapsulated packet
• A value of 1536 or greater in the Type/Length field indicates the type of the encapsulated packet
• The FCS (frame check sequence) in the Ethernet trailer is 4 bytes (32-bits) long
• CRC stands for cyclic redundancy check
• CRC is used to detect corrupted data by running an algorithm over the received data
• Cyclic refers to cyclic codes, redundancy refers to the fact that the 4 bytes at the end of the message enlarge the message without adding new information, and check refers to verifying the data for errors
• Ethernet frame's Frame Check Sequence is a Cyclic Redundancy Check
• MAC address is a 6-byte, or 48-bit, physical address assigned to a device when it is made
• MAC address is different from an IP address, which is assigned in the CLI when configuring the device
• MAC address is globally unique, no two devices in the world should have the same MAC address
• MAC address can also be locally-unique, not globally unique throughout the world