You already know from the previous chapter that routers function by creating a table of all networks it knows about. This table is called the routing table and routers use routing protocols to tell each other about the networks they know of. As networks increase, so do the number of entries in a routing table. Large routing tables cause increased processing and lower response time in a router. To reduce the size of routing tables, networks can be grouped together or summarized using a mask that incorporates them all. For example, in figure 2-5, a 192.168.10.0/24 subnet has been divided into smaller subnets of /27 mask. All of these networks connect to RouterA which it turn is advertising these routes to RouterB. Without summarization, RouterB will come to know of 8 networks which are available via RouterA. Since these networks are contagious subnets can have been subnetted from a /24 address, they can be summarized back into 192.168.1.0/24 network by RouterA while advertising to RouterB. This way, RouterB comes to know of one large /24 network only instead of 8 smaller /27 networks.