This page is all about Networks, which if you're viewing this page, you've already had experience in using the world's largest network, The Internet!
From this chapter you will learn the roles performed by computers on a network, distinguish between different network types, describe LAN networks, explain how computers connect to a network, how they communicate on a network, how network performance can be monitored, and the impacts of network failure.
Clients and Servers - Computers on a network usually act as one of two things; as servers (of which their are fewer), and clients (of which there tend to be more).
Servers are assigned responsibility for specific tasks on a network and provide services to other devices (clients). Examples of server functions include File servers, Application servers, and Print servers. A database server hosts large amounts of information and manages access to it. Security functions can also be performed by servers such as authentication servers (domain controller), for handling login requests. Servers can be dedicated firewalls if a network is connected to the Internet. Web servers store and serve web pages to users who connect to them. Email servers handle emails on a network if a third party is not used. Servers normally have a higher hardware specification (processor, RAM) than regular machines because they have to deal with many client requests at the same time. If a server is handling a lot of data it may have multiple hard disks offering many TeraBytes of storage, often arranged as a RAID (Redundant Array of Inexpensive Disks).
Client computers are regular desktop, laptops, or mobiles that connect to a server in order to use it's services. When connecting, clients normally have to authenticate themselves with a username and password.
Shared devices - Sharing of resources such as printers over a network has many benefits. For example, with printers, not every user needs to have their own printer, and costs can be reduced. Shared devices can be connected to a network hub or switch using Ethernet or USB, or they may be attached to a server or client computer.
Hubs, switches and routers - These devices all perform very similar task of sending data packets to travel to other places on a network, but the way they operate is different and affects the network performance and cost. The amount of available ports on devices varies and many of them also have WiFi capabilities built in too for wireless connectivity.
Hubs - Most basic and cheap. When a Hub receives data it broadcasts (sends) it to every device it is connected to (including the one that sent it). Computers ignore the data if it is not meant for them. Affects network performance as it uses a lot of unnecessary bandwidth sending the data over the network to devices it is not intended for.
Switches - Smarter version of Hubs. The inspect the data received, then only forward that data to the intended recipient. This saves a lot of bandwidth compared to a Hub.
Routers - Hubs and switches are used to connect multiple devices on the same Local Area Network(LAN). Routers have a slightly different task. They connect two or more separate networks. For example, a hub or switch might connect computers together on a school network, but a router will connect the school network to another network such as the Internet. The router acts as a gateway which all data coming in and out of the network passes through. This allows a router to perform additional tasks, such as filtering information that passes through (e.g. blocking certain websites being accessed from the network). Broadband routers are oftren supplied by your Internet Service Provider (ISP) and essentially combines the features of a router with the features of a switch. Meaning, it connects your home to the Internet, and allows multiple devices to connect to each other within your home (home network).
Network Architecture - How are computers logically organised on a network, and what role does each take? Client/Server is the most common architecture, servers provide services to the rest of the computers (clients). Centralised resources on a server reduces the load for client computers. On a thin-client network, servers not only handle things like files and printer sharing, but also run the software on the client's behalf. Thin client clients have just enough power to connect to the network and receive content from the server.
Peer to peer - On peer-to-peer networks every client or peer has the same status so there is no centralised authority or server. They are used in places like home networks where servers are not really necessary or too expensive. The concept of peer-to-peer is that computers share files from their own hard disk and other machines with the correct permission rights can access them.
They are limited in their ability to be useful once more than a few users are connected. Peer-to-peer networks shouldn't be confused with peer-to-peer file sharing tools (such as bittorrent) as they work differently. Peer-to-peer file sharing tools have a bad reputation as they are often used as a means to share illegally copyrighted material. The Napster case from 1999 meant that their system was shut down in 2001.
Firewalls - Hardware or software that determines what data is allowed to enter or leave a network. They prevent unauthorised access to a network and control which applications are allowed to connect to the Internet. Firewalls can block network traffic either by IP address, Domain names, Protocols & Ports, or by application. Governments in some countries such as China use firewalls to filter the Internet access for their citizens. You can check which countries are deemed the most filtered at Reporters Without Borders!
Proxy Servers - Proxy servers act as a go-between usually between a LAN and a web server on the Internet. All network traffic passes through the proxy allowing it to cache, filter and log data. They can be used by corporations to keep track of users' web activity. They can also be used to access content on the web that is banned on a particular LAN, as the proxy accesses the banned content on behalf of the user.
Caching is a process to help speed up web browsing. All content is stored temporarily by a proxy server in a place called a cache. When a user requests a site, the proxy checks for new versions of the files otherwise loads the locally stored version, speeding up the whole process and saving bandwidth.
Types of Network
- LAN - Local Area Network - Confined to a geographical area, like a home, a building or a campus.
- WLAN - Wireless Local Area Network - A LAN where devices connect wirelessly. Quite popular in homes.
- VLAN - Virtual Local Area Network - Connects geographically distant computers or LANs into one virtual network.
- PAN - Personal Area Network - When devices such as mobile phones and smartwatches connect within a few metres. Usually to sync devices or transfer data.
- MAN - Metropolitan Area Network - LANs connected together in a citywide scenario.
- SAN - Storage Area Network - Specialised network with storage devices connected to a LAN using high speed fibre connection.
- WAN - Wide Area Network - Large geographic area, multiple LANs connected together.
- VPN - Virtual Private Network - using a public network to create an encrypted tunnel from one location to a LAN somewhere. E.g. a businessman connecting to the Dubai office network to access their files.
Wired vs Wireless - Wired - Fibre Optic sending data via light at speeds of over 40 Gbps (Gigabits per second). Cable Internet access (DSL - Digital Subscriber Lines), use existing telephone lines and modems with ADCs (analogue to digital convertors). Dialup using telephone lines (old for 56.6kbps speeds), and Ethernet which are standard cables for most LANs. Data speed for Ethernet cables ranges from 100Mbps to 10Gbps speeds.
Wireless - WiMax is wireless technology designed to carry data over distances of up to 50km at around 50Mbps. WiFi (Wireless Fidelity) is a standard for connecting wireless devices in a small area. Bluetooth is a wireless technology for short distances, used in PANs for example. 3G or 4G (and 5G being tested) are wireless communications using mobile phone networks.
Communicating on a Network
MAC addresses - 'Media Access Controller' - A unique number built into most network devices typically stored in ROM. Each MAC address is unique. Often called a hardware address.
Protocols - A set of rules of how something should be done.
IP - Internet Protocol - This is how devices are identified on a network and how data can be sent between them. E.g. 192.168.0.1 is an IPv4 address. DHCP (Dynamic Host Control Protocol) helps allocate addresses and keeps a record of which device has which address. Private addresses are used to identify devices on private networks such as LANs with a public IP address given to the network gateway (often the router).
TCP - Transmission Control Protocol - Is closely related to IP. IP deals with the addresses of devices, while TCP handles ensuring data is sent and received correctly.
Ports - Ports identify services available on a network. For example, for HTTP which is for retrieving web pages, you will connect to port 80. HTTPS (secure) is port 443, SMTP (sending email) is port 25.
Future (IPv6) - IPv4 addresses have 4 octets like - 192.168.0.1. Meaning it's the 1st device on the 0th subnet, of the 168th subnet, of the 192nd subnet. Meaning total available addresses at one time is around 4.3 billion. With Web 3.0 and Internet of Things emerging, more and more devices need to be connected at one time. IPv6 fixes this by using hexadecimal address with a total combination of around 2128 addresses, which is more than enough for the predicted future.
Routing - Routing is the process of sending data from one network to another via the most efficient route. E.g. a web server may be located on the other side of the world, so to access it, data will need to pass through several networks. You can use Tracert in terminal or command prompt to see the steps data takes to reach your machine. Tracert www.google.com
DNS - Domain Name System - This translates URLs (Uniform Resource Locators or website addresses such as www.google.com) into IP addresses to enable you to connect to the website. URLs are obviously much easier to remember than IP addresses.
HTTP - HyperText Transfer Protocol governs communication between web servers and browsers. The encrypted version is HTTPS (secure). HTTPS uses TLS (Transport Layer Security) or SSL (Secure Sockets Layer) encryption to achieve the security.
Web on the Internet - To be clear, the Internet is the global network of LANs connected together. The web is the set of services that uses the Internet to send and receive data between those networks.
Web 2.0 + 3.0 - Web 2.0 was the beginning of user generated content on the web, such as blogs, wikis, social networks and Podcasts. Web 3.0 is the context of next step of the web, such as the Internet of Things (IoT) in which it is envisaged that everything is connected to the web; from your pens, to your house doors, to your clothes all communicating with each other and sharing data. Artificial Intelligence on the web is another possibility where people imagine a 'semantic web' that has metadata to link everything and help identify patterns and understand the relationship between everything.