类似地，网络外部性可以作为第四个原因，因为当一个产品的有用性随着使用它的人数的增加而增加时，它就会出现。手机就是一个很好的例子。在垄断市场中，边际收益曲线和需求曲线都是向下倾斜的。虽然边际收入和需求曲线向下倾斜，垄断市场赚取经济利润。在垄断市场中，生产过程发生在边际收入(MR)和边际成本(MC)相遇或相交的地方(Hubbard R. G.，利润最大化的垄断者，2016)。如图所示，需求与边际收入(MR)曲线向下倾斜。边际成本曲线(MC)高于平均总成本(ATC)。这里需求等于平均收入和价格。磁流变曲线位于需求曲线下。这个数字清楚地说明了垄断市场下的价格和数量均衡。它突出了垄断市场在竞争激烈的市场中，即使产量少但价格高，也能获得经济利润。很明显，与竞争市场相比，垄断者收取更高的价格，生产更少的产品。更高的价格由垄断者降低了消费者剩余,其中一些是被生产者剩余和一些导致无谓损失如下图描述,因此,上面的图了,价格、产量和利润的垄断市场可以通过检查需求,边际成本和边际收益(MR)曲线。垄断造成的经济效率低下，由于较高的价格和较低的产量。悉尼水务公司是一家在澳大利亚具有重要垄断地位的公司。它是一家国有企业，由新南威尔士州政府全资拥有。悉尼水务公司完全负责为家庭或工业提供各种用途的水。悉尼水务公司垄断了整个悉尼地区的水资源分配或供应(Sydney water, 2018)。它只是提供水管网络的公司，因为建立全国性的水管网络涉及很高的资本成本。
Similarly network externalities can be taken as the fourth reason as it occurs when the usefulness of a product increases with the number of people who use it. Mobile phones can be a good example.In case of monopoly market, marginal revenue curve and demand curve are downward sloping. Although marginal revenue and demand curve slope downward monopoly market earn economic profit. In monopoly market production process takes place at the point where marginal revenue (MR) and marginal cost (MC) meet or intersect each other (Hubbard R. G., Profit maximisation for a monopolist, 2016). It can be shown in figure as Above figure shows that demand and marginal revenue (MR) curve sloping downwards. Marginal cost curve (MC) lies above the average total cost (ATC). Here demand is equals to average revenue and price. MR curve lies under demand curve. This figure clearly illustrates price and quantity equilibrium under monopoly market. It highlights that monopoly market earns economic profit even by producing less but charging high price in the competitive market.It is clear that monopolist charges higher price and produces small quantity of goods as compared to competitive markets. The higher price determined by monopolist reduces the consumer surplus, some of which is captured by producer surplus and some results in deadweight loss as depicted by the figure below Thus, the above figure clarifies that the price, output and profit of monopoly market can be identified by examining the demand, marginal cost and marginal revenue(MR) curves. Monopolies cause economic inefficiency due to the higher price and the output that is lower.Sydney Water Corporation is a firm with significant monopoly power in Australia. It is a state owned corporation, wholly owned by the New South Wales Government. Sydney water corporation is fully responsible for supplying water for various purposes either for household or industrial purposes. Sydney water corporation has monopoly over the distribution or supply of water all over the Sydney region (Sydney WATER, 2018). It is only the firm providing the network of water pipes because there are very high capital costs involved in setting up a national network of pipes.