Internet Services and Protocols

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1Summary 重點摘要

The internet interconnects countless networks through the TCP/IP suite: IP addresses and forwards packets, TCP guarantees reliable ordered delivery, DNS resolves domain names into IP addresses, HTTP and HTTPS fetch web pages, SMTP and POP handle email, and FTP transfers files.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: The internet and why protocols matterThe internet is not one giant network but a network of networks interlinking countless smaller ones worldwide. For devices of different brands and countries to communicate, they must agree on common data formats and procedures — these rules are protocols. Without them, data sent by one side would be unreadable to the other. For example, a Hong Kong computer exchanging data with an American server works only because both follow the TCP/IP protocol suite, their common language.
2. Step 2: Division of labour between IP and TCPThe Internet Protocol handles addressing and delivery: it labels each packet with source and destination addresses so packets hop across networks. The Transmission Control Protocol, working above IP, guarantees quality: it numbers the data segments, waits for acknowledgements, and retransmits anything lost or out of order. For example, a large attachment is split into many packets taking different routes, and TCP reassembles them in sequence — IP manages the route, TCP manages reliability.
3. Step 3: The format of an IP addressAn IP address is a device's house number on the network. The common version, IPv4, has four numbers each from 0 to 255 separated by dots, such as 192.168.1.101; the newer IPv6 uses eight groups of hexadecimal digits, providing vastly more addresses for exploding device numbers. Addresses may be static or dynamic: servers usually keep static addresses so users can find them, while home devices get dynamic addresses assigned automatically. For example, 300.12.5.1 is invalid because 300 exceeds the limit of 255 — a common exam check.
4. Step 4: How DNS resolves a domain nameHumans remember www.example.com but not strings of digits; the Domain Name System is the translator between the two. After a web address is typed, the computer first asks a DNS server for the IP address matching the domain name, and only then sends the request to the target web server. Resolution is cached, so frequently used domain names are stored locally to speed up later visits. For example, when DNS fails, browsing by web address stops even though the network is fine, but entering the IP address directly still works — proof of its translating role.
5. Step 5: HTTP versus HTTPSThe web runs on a request-and-response model: the browser sends an HTTP request to a web server, which returns the page. Plain HTTP transmits in the clear, so content can be intercepted or even altered en route, and site identity is hard to verify. HTTPS adds an encryption layer beneath HTTP: data is encrypted before sending, unreadable even if captured, and the server's identity can be verified; the address starts with https. For example, payment and login operations involving passwords and card numbers must use HTTPS — a standard conclusion in data-response questions.
6. Step 6: How email protocols cooperateEmail is completed by two families of protocols in relay. When sending, the Simple Mail Transfer Protocol carries the mail from the sender's device to the sending mail server, and passes it between servers to the recipient's mail server. When receiving, the Post Office Protocol or similar downloads the mail from the receiving server to the recipient's device. For example, picture the postal system: SMTP is handing a letter to the post office and relaying it between offices, while POP is collecting mail from your post-office box to read at home — both are indispensable.
7. Step 7: FTP and other internet servicesThe File Transfer Protocol uploads and downloads files between client and server; administrators commonly use it to upload finished pages to a web server. Beyond file transfer, internet services include search engines locating information among billions of pages by keywords, cloud services offering online storage and software, streaming playing audio and video while still receiving, and instant messaging and forums supporting text, voice and video. For example, memorise services with their protocols: the web with HTTP, email with SMTP and POP — a reliable answering technique.

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5Worked example Example

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8Recap Recap

• Protocols are the common rules of network communication; the internet is built on the TCP/IP suite.
• IP handles addressing and delivery, TCP handles reliability and order — IP manages the route, TCP manages the quality.
• DNS resolves domain names into IP addresses, the translator between web addresses and numbers.
• HTTP sends in the clear while HTTPS adds encryption and verification; use HTTPS for anything involving passwords or payment.
• Email sends and relays with SMTP and downloads with POP; FTP handles uploading and downloading files.