Till this point we have assumed that people's demands for good are independent of each other.
Actually, a person's demand can be affected by the number of other people who have purchased that good.
If this is the case, a network exists externality.
Network externalities can be negative or positive.
A positive network exists externality if the quantity of a good demanded by a consumer increases in response to an increase in the purchases by other consumers.
Negative network externalities are the opposite of it.