Networks and the internet

To open a page, the browser takes the domain name, asks the DNS for the address, and sends a request to that server using HTTP. The server answers with the page written in HTML, and the browser draws it on the screen. To send an e-mail, your mail program hands the message to a mail server using SMTP. That server passes it on to the receiver’s mail server. The receiver reads it using POP, which usually brings the mail to his device (it may or may not leave a copy on the server), or IMAP, which lets him see and manage the mail kept on the server from several devices. How long does a download take? Speed is counted in bits per second, and 8 bits make one byte. Time = size in bits ÷ speed. In speed tasks we use decimal units: 1 MB = 1,000,000 bytes and 1 Mbps = 1,000,000 bits per second. For a 5 MB file at 8 Mbps: 5 MB = 40,000,000 bits, and 40,000,000 ÷ 8,000,000 = 5 seconds. Real downloads are slower, because the line is shared and packets are lost and resent.