1-1 CCNA

Simplex: BB machine, can only receive, not send
Half-duplex: walkie-talkie, can't send when receiving, can't receive when sending
Full duplex: telephone, mobile phone, can send and receive at the same time

Collision domain: When a node sends data to another node, in addition to the destination node, how many stations can receive the data, these stations constitute a collision domain.
Broadcast domain: If a site sends broadcast data, the range that can receive the data is a broadcast domain.
Unicast: One-to-one, with a clear source and destination
Multicast: one-to-many, or a group, data can only be received by joining the group, such as QQ group
Broadcast: One-to-all, usually the entire broadcast domain

Hub: All ports are in a collision domain, the whole machine belongs to a broadcast domain, and a physical layer device
Switch: each port has an independent collision domain, and the solicitation belongs to a broadcast domain
Router: each port belongs to independent collision domain and broadcast domain

"OSI is not a protocol, but a reference model"
- It is divided into 7 layers from top to bottom
  - Application layer: packaging of goods
  - Display layer: the name of the product
  - Session layer: the order number of the item
  - Transport layer: courier company
  - Network layer: Express order
  - Data link layer: mobile phone number
  - Physical Layer: Courier
Layers are independent of each other and cooperate with each other
When there is a problem at one level at a time, it will cause the data to not communicate normally

Port number: Different port numbers represent different application layer protocols and determine which transport layer protocol is used for transmission

MAC address: During transmission, it will change without going through a Layer 3 device
IP address: During transmission, it will not change unless a policy is made, such as NAT