A key concept in this book is a routing domain, depicted in Figure 1-7. The routing domain is an administrative entity, and its scope depends on many factors, but they are eventually determined by the network administrator, such as an ISP. The term scope means how many networks and subnets are associated with the domain. A small domain consists of a few subnets; a large domain consists of many. The size of the routing domain is relative, but its goal is to establish boundaries for the dissemination of routing information; in effect, to realize packet containment. If the domain contains many networks, it is likely that more routing packets must be exchanged than in a domain with fewer networks.