The single desktop...

Theoretically the simplest, this will usually be a simple application that only requires single user at a time access. The application relies on data stored locally on a single machine and does not require access to other services, either on a local area network or the internet. It is probably a small, specific-function application that performs and reports on a subset of the company's business function.

The Local Area Network (LAN)...

Here, data is required to be accessed by the application from two or more machines and to allow concurrent access to that data. The application will usually reside as a copy on each machine and will access data over the internal network, that data residing in a database on a server machine within the organisation. Considerations for user authentication and security are made based on user roles against relevant parts of the datastore.

The Wide Area Network (WAN)...

An organistaion may have several separate premises, which each require machines to connect to a data store (or stores) which may reside on one or more physical server machines located within the organisation. Transmission between sites is over high capacity lines or fibre-optic cable, although the method of transmission is not important from a software development standpoint. As with the case of the LAN, user authentication and security considerations are paramount.

The simple website

By this we mean a website consisting only of pages constructed with text and images, for example a brochure site. In this case, no data access via a database is required and therefore nothing on the site is generated dynamically: the pages are explicitly written and updated on an ad-hoc basis. The site is physically located with a third-party web-hosting provider company and a unique internet domain name must be aquired and registered to identify the site.

The Web Application

1. The Dynamic Website

This type of site may well, as with the previously described 'brochure' site, contain ad-hoc pages. In addition it will have access to a back-end datastore (again residing with a third-party hosting company), in order to dynamically generate pages based on user requests. The complexity level rises because of the need for a robust, scaleable database management system with which the website interacts.

2. The Intranet

An Intranet can be thought of by using the analogy of closed-circuit television. Although this type of site can be hosted on the internet with access only via authentication, it would usually be internally hosted, and not accessible to the general public via the internet at all. In every other way however, it is a website, but basically one that is to be used only by the company's employees. Like any other website, it may be designed just with static pages or it can be fully dynamic.

3. The Extranet

An Extranet is hosted in the same manner as above and, often, secured so as to be innaccessible to the general public. However, access may be required by authorised people external to the business, for example customers and suppliers. Each of these external groups will be authorised only to access areas of the site specifically designated to them. This type of website is far more likely to employ back-end datastores to generate dynamic information and allow for addition and updating of data.

It is entirely possible, depending on the nature of the site, that there will be a necessity for pages to which the general public has access via the internet and in this case the line between an Extranet and the web in general become rather blurred!

In this case it is also more likely to comprise sections which form a complete application; for example, an airline's on-line booking reservation system.