A variety of Ethernet speeds and grades
| name | speed | Network cable level |
| Ethernet | 10Mbps | - |
| Fast Ethernet | 100Mbps | CAT 5 |
| Gigabit Ethernet | 1000Mbps | CAT 5e/CAT 6 |
Two-cross line and straight line
- Crossover cable: When one side is 568A and the other side is 568B connector, it is called crossover cable, which is used to directly connect the network card of two hosts.
- Straight cable: When the connectors on both sides are 568A or 568B, it is called a straight cable, which is used to connect the cable between the host network card and the hub.
3. Automatically distinguish the crossover or direct connection of the network cable (Auto MDI/MDIX)
Do we need to pay attention to whether the cables being connected are crossover or straight? unnecessary. Because the Switch has the Auto MDI/MDIX function, it will automatically distinguish the connector of the network cable to adjust the connection, so you don't need to check whether the network cable is a crossover cable or a straight cable.
Four types of IP
1 IPv4 has only two IP classes
- Public IP: The public IP, the IP planned by the Inter NIC, can be connected to the Internet only with this IP.
- Private IP: Private IP or reserved IP, which cannot be directly connected to the Internet. It is mainly used for host connection planning in the LAN.
2 Private IPs are reserved for each of the three categories A, B, and C, which are:
Class A：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
Five ways to obtain IP
1 Direct manual configuration (static)
You can directly ask your network administrator for available IP-related parameters, and then directly edit the configuration file (or use some software) to configure your network. Commonly used in the campus network environment and the connection environment where a fixed IP is applied to the ISP.
2 Obtained by dialing
Apply for registration with your ISP, and after obtaining the account password, dial directly to the ISP. Your ISP will use their own settings to allow your operating system to obtain the correct network parameters. At this point you do not need to manually edit the network parameters related to the configuration.
3 Automatically obtain network parameters (DHCP)
There will be a host in the local area network responsible for managing the network parameters of all computers. When your network starts, it will actively request IP parameters from the server. network parameters. Most commonly used within enterprises, IP router backends, campus networks, and dormitory environments.
Correspondence table of six common ports and network services
| port | Service name and content |
| 20 | FTP-data, the active data transfer port used by the file transfer protocol |
| 21 | FTP, File Transfer Protocol Command Port |
| 22 | SSH, a more secure remote connection service |
| 23 | Telnet, an early remote connection service software |
| 25 | SMTP, Simple Mail Delivery Protocol, used as a port for Mail Server |
| 53 | DNS, used as a name server for name resolution |
| 80 | WWW, the World Wide Web Server |
| 110 | POP3, the mail receiving protocol, the mail receiving software used in the office is all through it |
| 443 | HTTPS, an encrypted WWW server |
Seven TCP three-way handshake
TCP is known as a reliable data transmission protocol, the most important of which is the three-way handshake function. The packet connection mode of the three-way handshake is as shown in the figure below.
A: Packet sending
When the client wants to connect to the server, it must send a data packet requesting a connection. At this time, the client must randomly select a port greater than 1024 as the interface for program communication. Then, in the TCP header, there must be an active connection with SYN (SYN=1), and note the sequence number (Seq=10001) of the connection packet sent to the server.
B: Packet reception and confirmation of packet delivery
When the server receives the data packet and confirms receipt of the data packet, it will start to make a data packet with SYN=1, ACK=1, in which the number of ack is to be used by the client for confirmation, so the number will be There is one more Seq than the Seq in step A. At the same time, our server must also confirm that the client can indeed receive our data packets, so it will also send Seq=20001 to the client and start waiting for the client to serve us. end response.
C: echo acknowledgement packet
When the client receives the ack from the server, it can confirm that the previously sent data packet has been received, and then if the client also agrees to establish a connection with the server, it will send a confirmation packet ACK=1 to the server again. ack=20002.
D: get final confirmation
If all goes well, the connection can be established after the server receives the data packet with ACK=1 and ack=20002 sequence number.