Overview of Networks

Two or more connected computers that communicate (e.g. E-mail) and share resources e.g. files, peripherals.

Individual computers are called nodes. Resources are stored on/controlled by a server, which gives access to less-powerful client computers. Servers include: File servers, Print servers, Network serves, E-mail servers, Web servers.

Local Area Network (LAN). Network in confined geographic area, usually owned by a company.
Wide Area Network (WAN). Wide geographic area, crossing one or more public highway, usually rented.

Origins of Internet:

• 1960s – developed by American military: ARPAnet.
• Messages split into Data Packets which makes communication less dependent on individual nodes.

Packet: Data processed by a protocol so that it can be sent across a network.

A website (e.g. www.mysite.com) contains several individual web pages (e.g. www.mysite.com/first.htm, www.mysite.com/second.htm).

Protocols are rules that describe how clients and servers communicate across a network.

Remote access protocols allow you to connect to a network remotely (e.g. from home)

• Point-to-Point Protocol (PPP) used by most dial-up systems.
• Point-to-Point Protocol over Ethernet (PPPoE) often used by LAN systems with Ethernet.

Connecting to the Internet requires 6 things:

• Computer
• Operating system (e.g. Windows, UNIX, Linux)
• TCP/IP software (usually comes with computer)
• Client software such as web browser, E-mail client
• Internet connection, usually via an ISP
• Internet address

Any request for data from a web server through a LAN:

1. Request is divided into data packets.
2. Packets are routed through the LAN to the Internet backbone.
3. Packets are routed through the backbone to the server.
4. Server responds with data, although it may be via a different route.

N.B. Web-based E-mail (such as Hotmail) does not use SMTP, IMAP or POP3. Since it is based on web pages, it uses the HTTP protocol (and Port 80).

Each device (N.B. not just computers) on the net has an Internet Protocol (IP) address that identifies it uniquely in the world. These devices are also referred to as hosts.

Dial-up Internet connection: Net connection via phone lines connected to the computer through a modem.
Baud speed (kbaud = Kilobytes per second): 14.4 kbaud to 128 kbaud. Generally SLOW
Integrated Services Digital Network (ISDN): Digital telephone line, FAST and expensive.

Domain names

Basically, a web address e.g. www.mysite.com

A Fully qualified domain name (FQDN) gives enough information to convert the domain name into an IP address. Must include the three parts listed above (so www.mysite.com does count as an FQDN)

Root-level servers are the highest level of the domain name system e.g. www.

Top level domains:

com mercial	edu cational	org anization	mil itary (US)
gov ernment (US)	net works	int ernational organizations
aero = travel industry	biz = business	coop eratives	museum
name = personal web address	pro fessional	info = content and research related
au stralia	ca nada	ch = Switzerland	dk = Denmark
fr ance	gr eece	jp = Japan	mx = Mexico
uk = United Kingdom		us = United States

Domain name server: A server that translates IP addresses into web addresses (e.g. 12.42.192.73 into www.ciwcertified.com). Reverse DNS involves translating the other way.

A Virtual domain server hosts several domains. It allows domain names to be shorter e.g. my web site is hosted on Tripod, so its name is www.tripod.com/richardbowles.tripod.com. However, the Tripod server allows you to type richardbowles.tripod.com.

Backbone: The highest level in the computer network hierarchy to which smaller networks typically connect.

Bandwidth: The amount of information (“traffic”) that can be transmitted across a connection at one time.

Client software: Web browser, E-mail or news client program.

Dead link: Link sending the user to a non-existent link (e.g. “404 error”)

Gateway: A node on a network acting like a portal to other networks.

Internet Protocol version 4 (IPv4), or Dotted Quad (Dotted Decimal Notation): URL in the form of four numbers, each 0 to 255, linked by dots e.g. 12.42.192.73

Internet Protocol version 6 (IPv6): URL extended to 8 4-digit hex numbers.

Internet Service Provider (ISP): Organisation providing connection to the Internet. Most charge, some free.

Router: routes packets based on their network-layer addresses

Wireless connections:

• Hybrid network, in which a Wireless Access Point (enabling wireless systems to connect with each other) is connected to a cable-based Internet connection.
• A standard system using a wireless network interface card (NIC) is called an end-point.
• There are two types of wireless connection, ad-hoc (systems only use their NICs to connect) and infrastructure (connections is via a centralised WAP).
• 802.11 standard governs a family of specifications for wireless technology.

Standard: A format or definition approved by some recognised standards authority.

Channel: A cable or signal between two nodes that allows communication.

T and E carriers:

• Dedicated digital telephone lines. T refers to US lines, E to European equivalents.
• Lines are expensive and are leased typically from ISPs.
• Companies can lease single channels on T or E lines (cheaper) - "fractional T and E lines"

Direct Connection Speeds:

Connection type	Speed
Fiber-optic cable	up to 100 Giga bits/second)
T1 line	1.544 Mega bits/sec. Typically used by American corporate LANs to connect to ISPs
T3 line	44.736 Mega bits/sec. Typically used by American corporate LANs to connect to the Internet.
E1 line	2.048 Mega bits/sec
E3 line	34.368 Mega bits/sec
Cable modem	From 512 kilo bits/sec to 52 Mega bits/sec
xDSL modem	From 512 kilo bits/secto 32 Mega bits/sec

Digital Subscriber Line (DSL):

• High-speed connection using digital phone lines and xDSL modem.
• Variations include xDSL, Adaptive DSL (ADSL) and Rate-Adaptive DSL (RADSL).

Connecting cable and DSL modems:

• Modem connects a personal computer to the Internet.
• Network cables used to connect DSL modems to the wall. Ethernet cables with an RJ-45 connector at each end.

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