TCP/IP is the family of communication protocols used as the standard on the Internet, combining several protocols with different roles by dividing them into layers (階層).
Layer
Main role
Example protocols
Application layer
Defines the exchanges for each use, such as web browsing and email
HTTP, HTTPS, SMTP, POP3, FTP
Transport layer
Passes data to the destination application
TCP, UDP
Internet layer
Relays data to the destination device by IP address
IP
Network interface layer
Actually sends signals within the same network
Ethernet, wireless LAN
A communication protocol (通信プロトコル) is an agreed set of procedures and formats for exchanging data over a network. TCP/IP is usually described in four layers: the application layer, the transport layer, the Internet layer, and the network interface layer. The two protocols in its name split the core work: TCP delivers data reliably with nothing missing or duplicated, and IP carries it to the destination device based on the IP address.
AnalogyThink of a delivery company where intake, sorting, and transport are handled by separate departments, and each one hands parcels only to the department next to it. If one department changes how it works, the others can carry on as before.
How to rememberThe name joins the two core protocols, TCP and IP, with a slash. Used as the name of the whole family, it also covers many other protocols.
Exam points
ExamA communication protocol (通信プロトコル) is a set of rules for communication, and devices that follow the same rules can exchange data even if their products or operating systems (OS) differ.
ExamThe name TCP/IP refers to the whole four-layer family, and in a narrow sense to just its core, TCP and IP. In that narrow sense, TCP/IP is the lower-level communication function that protocols for specific uses all rely on.
UDP is a connectionless transport-layer protocol that sends data without setting up a connection with the other side.
TCP
UDP
Connection before sending
Set up first, then send
Send right away, no setup
Delivery check and resend
Yes
No
Suited to
Transfers where no data may be lost
Voice and video that cannot wait
Because it neither checks whether data arrived nor resends lost data, it involves fewer steps and less delay. If missing data or out-of-order arrival must be dealt with, the application handles it. It suits traffic such as voice calls and video streaming, where avoiding waits matters more than losing a little data.
AnalogyIt is like a public announcement in a building that keeps playing without waiting for any response. If someone misses it, it is not replayed just for that person.
How to rememberIts full name is User Datagram Protocol. A datagram is a small packet sent off on its own, and the name reflects this send-and-forget nature.
Exam points
Mix-upUDP is a transport-layer protocol just like TCP, and both use port numbers (ポート番号) to tell destination applications apart.
A port number (ポート番号) tells which of the communication applications running on one computer the traffic is for.
The sender attaches the destination IP address and port number
The IP address gets it to the destination device
The port number hands it to the application inside
Next, the port number decides which application in that device receives it.
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An IP address pinpoints a device on the network, while a port number pinpoints the destination application inside that device. Port numbers are handled by TCP and UDP and range from 0 to 65535. Common services have numbers assigned in advance, called well-known ports: HTTP uses 80 and HTTPS uses 443, for example.
AnalogyIt is like mail delivered to a company's address being sorted onto each department's shelf by the department name on it. The address is the IP address, and the department name is the port number.
How to rememberPort means harbor. Ships (data) may reach the same island (device), but the harbor they enter decides which town (application) they go to.
Exam points
ExamA port number (ポート番号) identifies a communication application running inside a device, a different job from the IP address (IPアドレス), which identifies a device on the network.
ExamEven when one server (サーバ) offers several services (サービス), users can pick the service they want by specifying its port number.
HTTP is the application-layer protocol a web browser and a web server use to exchange web page data.
The browser asks for a page
The server returns the page data as a response
The browser displays what it received
The browser displays the data it received, and one round trip is complete. Images and other files are fetched by repeating the same round trip.
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The browser sends the page it wants as a request (リクエスト), and the server returns the content as a response (レスポンス); communication proceeds through these round trips. In HTTP/1.1 and HTTP/2, reliable delivery of the data is left to TCP at the transport layer, and port 80 is the usual number.
AnalogyIt is like ordering a dish at a restaurant and having it brought to your table. Each order gets one serving in return.
How to rememberThe Transfer in HyperText Transfer Protocol means moving data. Break the name down and it reads as a procedure for transferring linked documents (hypertext), which is exactly its job.
Exam points
ExamHTTP belongs to the application layer (アプリケーション層) and defines how a browser and a server (サーバ) interact when you view a web page.
Mix-upHTML is a language that describes the content and appearance of a web page, while HTTP is the communication procedure that carries that data; the names look alike but the roles differ.
HTTPS is a protocol that encrypts HTTP traffic with SSL/TLS and also lets you check that the server you are talking to is genuine.
SSL/TLS is the mechanism that encrypts communication and authenticates servers using digital certificates. It is widely used on pages that handle information others must not see, such as passwords and credit card numbers. The URL starts with "https://", and port 443 is the usual number.
AnalogyIt is like moving messages from postcards into sealed envelopes. People handling them on the way cannot see inside, and the envelopes from the shop (the server) carry an officially certified seal, so the customer (the browser) can confirm the shop is genuine.
How to rememberThe final S stands for Secure. The name just adds one letter to HTTP, and those five letters also open the URL.
Exam points
ExamHTTPS is used to secure web traffic: SSL/TLS authenticates the server (サーバ) and encrypts (暗号化) the content exchanged.
ExamSwitching from HTTP to HTTPS makes the traffic harder for third parties to read, which preserves confidentiality (機密性).
ExamHTTPS encryption covers both what the browser sends and what the server sends back.
A communication protocol is an agreed set of rules for exchanging data, and TCP/IP is the Internet-standard family that organizes such rules into four layers.
UDP sends data without setting up a connection or checking delivery, and is used where low delay matters.
The IP address takes data to the right device, and the port number takes it to the right application inside.
HTTP is the application-layer protocol that carries web pages through request-response round trips.
HTTPS uses SSL/TLS to add encryption in both directions and server authentication to HTTP.