Network Basics 01

• UDP header is 8 bytes, TCP header is 20 bytes
• The broadcast domain is the concept of the second layer in the OSI model, so Hub (multi-port repeater), repeater, switch, bridge, etc., the nodes connected to the second layer devices are considered to be in the same broadcast domain . Only Layer 3 switches such as routers can divide broadcast domains.
  The collision domain is the concept of the first layer in the OSI model. The devices connected to the same collision domain include Hubs, repeaters or other devices that simply replicate signals. All nodes connected by a hub or repeater can be considered to be in the same collision domain. Layer 2 devices (bridges, switches) and Layer 3 devices (routers) can divide collision domains.
• The network ID of class A address occupies 8 bits, the highest bit is 0; the network ID of class B address occupies 16 bits, and the highest two bits are 10; the network ID of class C address occupies 24 bits, and the highest three bits are 110.
• The switch forwards based on the destination MAC address
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• According to the transmission mode of light in the fiber, it can be divided into: single-mode fiber and multi-mode fiber. Multi Mode Fiber: The central glass core is thicker (50 or 62.5 μm), which can transmit light in multiple modes. Single-mode fiber (Single Mode Fiber): The central glass core is very thin (the core diameter is generally 9 or 10 μm), and only one mode of light can be transmitted.
• Reserved IPs: 1 class A address 10.*.*.* 16 class B addresses 172.16.*.*---172.31.*.* 256 class C addresses 192.168.0.*---192.168.255. *
• Class A IP addresses range from 1.0.0.0 to 126.0.0.0. There are 126 Class A networks available. 
  Class B IP addresses range from 128.0.0.0 to 191.255.255.255. There are 16,382 Class B networks available. 
  Class C IP addresses range from 192.0.0.0 to 223.255.255.255. Class C networks can reach more than 2.09 million. 

• Private address (available on the internal LAN)

  A-level: 10.0.0.0 - 10.255.255.255

  Class B: 172.16.0.0 - 172.31.255.255

  Class C: 192.168.0.0 - 192.168.255.255

  Reserved address (special purpose)

  Class A: 127.XXX

  Class B: 169.254.XX