All proxy servers contain two parts divided with column - 163.29.225.250:8080. The first part - 163.29.225.250 is the IP address of the computer (server) where proxy server is located while the last 4 digits are so called port. If you are reading this article than it means that you are interesting what port is in general and what it was designed for?
Personal computer (PC) which is connected to the Internet or local area network (LAN) has its own unique IP address. Basically that IP address is used by different programs running on PC to find another computer which is also connected to the Internet or LAN. The simplest way to understand how port works and it is purpose of usage is to make parallel with real world.
First - lets imagine that PC is like a big building with 10 flats inside (for different families). Each flat in that building is like a separate program running on your PC. Finally the IP address will be building's physical address. (what will be a port, I will explain a bit later...)
Second - let's imagine the situation when someone sends parcel to our address BUT forgets to write a flat number.
As you already guess the parcel could be delivered to the building but after that post officer will have a problem to identify the receiver. There are 10 different families living in 10 different flats in that building (on one IP) and anyone could be an owner. To prevent such situation we have to specify not only building's address but also a flat number.
Similar to the example described above in the Internet all network parcels (data) should be sent with IP address (building's address) and port number (flat number). There could be dozen of different programs (flats) running on one PC (big building) but each of them MUST have their own port (flat number) to receive network parcels.
Port number could get values from 0 up to 65535. BUT please take in account that all ports from 0 till 1024 are reserved for some well known services. For example: port 80 is defined as a default for HTTP protocol (to open websites), 110 for POP3 (to receive emails), 25 for SMTP (to send emails), 21 for FTP (to upload files on the server) and so on. The rest are used dynamically by other programs for each connection.
As a conclusion: every program which wants to receive requests from other computers MUST select one port. Please take into account that only ONE program at a time could use ONE particular port for receiving network parcels. Otherwise it would be impossible for operating system (post officer) to identify the receiver, of network parcel. In other words, it is exactly why your system generates error message "this prot is already in use" and required reboot.
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