Networking and the Internet

Network types

A computer network is two or more devices connected to share data and resources.

An intranet is a private network for an organisation's own staff; an extranet opens part of it to chosen outsiders.

Topologies

• Bus: all devices share one backbone cable; a break in the backbone stops the network.
• Star: every device connects to a central hub or switch. One faulty cable affects only that device, but failure of the central device stops everything.
• Ring: devices form a closed loop and data travels around it.
• Mesh: every device has a direct link to every other device. A full mesh of n devices needs n(n - 1) / 2 links, so 5 devices need 5 x 4 / 2 = 10 links.
• Tree and hybrid: combinations of the above.

Network devices

IP address and DNS

• IPv4 addresses are 32 bits long, written as four decimal numbers (octets) separated by dots, for example 192.168.1.10.
• IPv6 addresses are 128 bits long, written in hexadecimal groups separated by colons.
• DNS (Domain Name System) translates domain names such as nrb.org.np into IP addresses; it is called the address book of the Internet.
• DHCP assigns IP addresses to devices automatically.

Protocols and their ports

TCP is connection-oriented and reliable; UDP is connectionless and faster but does not guarantee delivery. TCP/IP is the protocol suite of the Internet.

Email and the web

• An email address has the form user@domain. CC sends a visible copy; BCC sends a copy whose recipients are hidden from the others.
• The World Wide Web is a service that runs on the Internet; the two are not the same. A browser displays pages written in HTML, each located by a URL. A search engine finds pages by keyword.

Cloud computing

Cloud computing delivers computing resources over a network on demand, paid for by use. The NIST definition gives three service models and four deployment models.

• IaaS (Infrastructure as a Service): rented servers, storage and networks.
• PaaS (Platform as a Service): a ready platform on which developers build and deploy applications.
• SaaS (Software as a Service): finished software used through a browser, such as web-based email.
• Deployment models: public, private, community and hybrid cloud.

Exam pointers

• LAN, MAN, WAN are ranked by geographic coverage, not by speed or number of computers.
• A hub broadcasts, a switch uses MAC addresses, a router uses IP addresses.
• IPv4 is 32-bit, IPv6 is 128-bit, a MAC address is 48-bit.
• SMTP sends mail; POP3 and IMAP receive it.
• Memorise the ports: 21, 22, 23, 25, 53, 80, 110, 143, 443.
• Full mesh links = n(n - 1) / 2.
• Web-based email is SaaS; a rented virtual server is IaaS.