network security firewall

== Some common ports ==
HTTP: port 80
FTP: port 20,21
In the TCP/IP protocol, the link between the transport layer and the application layer is realized by ports.A port is actually a virtual thing.
Not only the process receiving the datagram needs to open its own port, but also the process sending the datagram.

== Port concept == In network technology, port has roughly two meanings: one is a port in the physical sense, such as an interface used by ADSL Modem, hub, switch, and router to link other network devices.The second is the port in the logical sense, which generally refers to the port in the TCP/IP protocol. The port number ranges from 0 to 65535, such as port 80 for browsing web services and port 21 for FTP services.

Port classification
According to the distribution of port numbers:
(1) Well-known ports
Well-known ports are well-known port numbers, ranging from 0 to 1023. These port numbers are generallyAllocated to some services.For example, port 21 is assigned to the FTP service, port 25 is assigned to the SMTP (Simple Mail Transfer Protocol) service, port 80 is assigned to the HTTP service, and port 135 is assigned to the RPC service.
(2) Dynamic ports
The range of dynamic ports is from 1024 to 65535. These port numbers are generally not fixedly assigned to a service, that is, these ports can be used by multiple services.As long as the running program requests the system to access the network, the system can allocate one of these port numbers for the program to use.For example, port 1024 is assigned to the first program that sends an application to the system.After closing the program process, the occupied port number is released.
However, dynamic ports are often used by virus Trojan programs, such as Glacier's default link port is 7626 and so on.

Divided by protocol type:
(1) TCP port
The TCP port, the Transmission Control Protocol port, needs to establish a connection between the client and the server, which can provide reliable data transmission.Common ones include port 21 for FTP service, port 23 for Telnet service, port 25 for SMTP service, and port 80 for HTTP service, etc.
(2) UDP port
UDP port, that is, the user data packet protocol port, does not need to establish a connection between the client and the server, and the security is not guaranteed.It is common to have port 53 of the DNS service and so on.
** Firewall Types **
Firewalls can be divided into software firewalls and hardware firewalls.Software firewalls can be further divided into personal firewalls and gateway firewalls.
== Personal Firewall: == Personal Firewall runs on the PC and is used to monitor the communication information between the PC and the external network.Windows Firewall is integrated into the Windows operating system.

== Gateway Firewall == The function of configuring the firewall on the gateway in the network can carry out policy control on the traffic in the network, which is the gateway firewall.
There are two types of gateway firewalls, one is a software gateway firewall that installs and runs firewall software on Windows, Linux and other operating systems, and the other is a hardware gateway firewall that uses special equipment.
== Personal firewall mainly monitors the communication flow of PC, and gateway firewall monitors the communication flow of all terminals in the network, and carries out policy control at the gateway.==

== What threats does the firewall protect against?==

1, Eavesdropping: Obtain important information such as bank card numbers and passwords by eavesdropping on network data.
2. Tampering: maliciously modify the website homepage, mail and other communication content
3. Destruction: destroying the normal work of the system through computer viruses or Dos attacks
4. Impersonation: pretending to be others to send emails,phishing, scams and other news to the party
5. Information leakage: Disclosure of important information or documents on the computer or server
6. Attack shift: as a springboard for viruses or DoS attacks
7. Spam: mass mailing for profit

** Firewall **
A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.All network traffic to and from this computer passes through this firewall.The firewall scans the network traffic flowing through him.This filters out some attacks from being executed on their computer.Firewalls can also close unused ports, block outgoing traffic from specific ports, and block access from specific sites, preventing all communications from unknown intruders.