How does the company block some network access (want to know how)?

it is understandable for the company to block some websites for security and other reasons.
would like to know how to block it.
now the company"s network can be connected to the external network, but it is blocked, such as

Taobao and other shopping sites;
QQ, Wechat, Zhihu and other social software traffic (after the mobile phone is connected to the company network, QQ and Zhihu can receive a message reminder, but after clicking in, they will not be able to see the new message, and Wechat will not receive a message reminder).
all kinds of cloud services, such as Baidu Cloud, iCloud, etc., cannot be accessed on the web page and cannot be connected to the client.
I rented a VPS with SSR, to connect but cannot surf the Internet;
you can ping your own VPS, but you cannot connect using SSH.

these are probably the cases in which they are not available on the Internet.
would like to know how the company blocks these operations.
Thank you

Firewall network-communication network-security

Mar.21,2021

you should only be able to visit certain websites that are on the whitelist. Using the ancient Chinese city walls to enter and enter the customs, if the people in the city want to leave the city, they have to go through cross-examination. If the local officials stipulate which people are not allowed to enter or leave, there will be a collection of portraits at the city gate for comparison, and the people who are on the number will not be able to get out. By the same token, now the local officials say that people in the portrait collection can go in and out, so people who don't match the portraits can't get in and out. Back on the network, there is a whitelist (photo collection) in the company's top-level routing (city gate duty officer). This whitelist is responsible for domain names, ip, ports (people entering and leaving) other than Filter blocking list. Call back all domain names or ip that are not in the whitelist and do not give any ack feedback, then tcp handshake fails and the connection is interrupted when establishing the connection. People in the city can't get out and people outside the city can't get in.

configure whitelist or blacklist according to the common port protocol of domain name. It may be firewall or DNS pollution

you can be convinced.

this kind of company should close down before it is too early,

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