Network: IP address and DNS

The IP address was first designed in the 1970s when the population of Earth was 4 billion, so there are about 4.2 billion IP addresses that can be made. The number of people now is over 7 billion, which IPv4 lacks to distribute all the addresses to everyone on Earth. As an alternative, there has been IPv6 that has 16 bits in each octet instead of 8. Moreover, It can make an almost infinite number of IP addresses, so now, we don't have to worry.

To see your IP address, you could enter 'ipconfig' on Command Prompt.

Ip address is designated based on your location. It has xxx.xxx.xxx.xxx format, and each 'xxx' is called "Octet" since each octet is filled with 8 bits(in total 32 bits.).

The IP address consists of numbers that are very hard to remember. We would instead remember 'Domain Names', which are easier to remember because they are in 'Human languages'. This system is called 'DNS(Domain Name System)'. If you want to know the IP address of a particular domain, you will use the keyword of 'ping' on the Command Prompt.

To inspect, the system sends 4 packets to the website to see if the server works well. Some servers block people from seeing their IP addresses. Moreover, we can just copy the IP address and paste it into the address bar to enter the same website.

To connect server and client, you need to know 1)the IP address of the server and 2)the number of the port. Following is a table of relatively well-known port numbers.

Port No.	Description
21	ftp(File Transfer Protocol)
22	ssh(Secure Socket Shell)
23	telnet(Teletype Network Protocol)
80	http(Hypertext Transfer Protocol) : Usually omitted.
25	smtp(Simple Mail Transfer Port)
110	pop3(Post Office Protocol 3)