Internet and Its Applications: Protocols, Addressing and Services
Internet and its applications tests how networks exchange data, how addresses and names work, what common protocols do, and how web, email and cloud services are used. Questions often ask you to interpret a URL or choose the right protocol.
Internet, WWW and packet switching
The internet is a global network of networks that uses TCP/IP. The World Wide Web is one service on it: hyperlinked resources requested with HTTP/HTTPS. Data is split into packets, each carrying source and destination addresses and sequence information. Routers forward packets independently, and the destination reassembles them; packets may take different routes.
Addressing and DNS
IPv4 addresses are 32-bit values written as four decimal octets; IPv6 uses 128 bits in hexadecimal. An IP address identifies an interface, while a domain name is human-readable. DNS is a hierarchical system that resolves domain names to IP addresses, so a browser can contact the right server. A URL has the form scheme://host[:port]/path?query#fragment.
Protocols and services
HTTP is a request–response protocol; HTTPS adds TLS encryption and integrity protection. FTP transfers files. For email, SMTP sends messages, while POP3 and IMAP retrieve them; IMAP keeps mail on the server and synchronises devices. Cloud computing provides on-demand computing resources, commonly as IaaS, PaaS or SaaS. Bandwidth measures data rate; latency measures delay.
Worked example
For https://www.example.edu.hk:443/apply/form.html?id=7#step2, the scheme is https, host is www.example.edu.hk, port is 443, path is /apply/form.html, query is id=7 and fragment is step2. DNS resolves the host to an IP address; the browser connects to port 443 and sends an HTTP request for the path and query. The fragment is handled by the browser, not sent as part of the HTTP request.
Traps
The internet and the WWW are not synonyms. DNS normally maps names to IP addresses, not the reverse. SMTP does not retrieve mail. HTTPS does not guarantee a site is honest-it protects the connection. Do not confuse the path with the query string, and remember IPv4 is 32-bit.