Computers around the world with different manufacturers and operating systems can communicate because they follow common “rules.” These rules are called protocols (communication standards). The collection of protocols that supports the Internet is TCP/IP. In this lesson, you will learn about the layered structure of TCP/IP and how IP addresses and DNS work.
Protocols and Layering
Communication requires many rules, such as “What format should the data have?”, “How should it be delivered to the other party?”, and “How should errors be handled?” TCP/IP divides these functions into four layers according to their roles.
・Application layer: Rules for each service, such as HTTP/HTTPS for browsing the Web, SMTP for sending email, and POP/IMAP for receiving email.
・Transport layer: Responsible for delivering data reliably. TCP checks whether data has arrived, retransmits missing packets, and puts them in order.
・Internet layer: IP manages destination addresses, determines packet routes, and carries the packets.
・Network interface layer: Responsible for physical transmission through cables and radio waves, such as Ethernet and Wi-Fi.
Layering has the advantage that each layer can be improved or replaced independently. For example, the upper layers can continue to be used even if a wireless connection is changed to a wired connection.
IP Addresses
The number used to identify a device on a network is an IP address. The widely used IPv4 uses 32 bits and is written as four groups of 8 bits, such as “192.168.1.1.” The number of addresses that can be represented is 232＝approximately 4.3 billion, but because this became insufficient with the spread of the Internet, migration to IPv6, which uses 128 bits (approximately 3.4×1038 addresses), is progressing. There is also a distinction between a unique global IP address on the Internet and a private IP address that is valid only within a home or school LAN.
DNS — The Directory Matching Names with Addresses
Because a string of numbers such as “192.0.2.44” is difficult for people to remember, domain names such as “example.com” are used. The system that matches domain names with IP addresses and converts between them is DNS (Domain Name System). When you enter a URL in a browser, it first contacts a DNS server to obtain the IP address, and then communication begins with that address.
① Ask the DNS server, “What is the IP address of example.com?”, and obtain the IP address.
② Establish a connection with the other server using TCP.
③ Send a request using HTTP(S): “Please give me this page.”
④ The server returns the page data as a response, and the browser displays it.
During this process, the data is divided into packets, carried along routes by IP, and reassembled completely and in order by TCP.
- Protocol＝A rule for communication. TCP/IP is the standard group of protocols for the Internet.
- Four layers: Application (HTTP・SMTP, etc.)／Transport (TCP)／Internet (IP)／Network interface.
- TCP is responsible for “reliably and in order,” while IP is responsible for “carrying data to the destination.”
- IPv4 uses 32 bits and provides approximately 4.3 billion addresses. Because this is insufficient, migration to IPv6 (128 bits) is underway.
- DNS converts between domain names and IP addresses; it is the “telephone directory of the Internet.”
Practice Problems
- State how many bits an IPv4 IP address uses and approximately how many addresses it can represent.
- Explain the role of DNS.
- Explain the difference between the roles of TCP and IP.
Answers and Explanations
- Answer：32 bits, approximately 4.3 billion
Explanation：2 to the 32nd power＝approximately 4.29 billion. Because there are not enough addresses, migration to 128-bit IPv6 is progressing. - Answer：A system that matches domain names (such as example.com) with IP addresses and converts between them.
Explanation：It serves as a bridge between names that are easy for people to remember and numerical addresses used by machines. When opening a URL, a DNS inquiry is made first. - Answer：IP uses IP addresses to carry packets to their destinations (routing). TCP checks whether packets have arrived, retransmits lost packets, puts them in order, and delivers the data reliably.
Explanation：It is useful to organize the roles as “IP carries the data, and TCP guarantees its quality.”