ks, as long as they share a common network prefix. Basically, with CIDR, a route is 199.182.20.17 199.182.20.16 199.182.20.5 Router 128.10.30.1 199.182.20.1 199.182.20.0 203.16.25.100 128.10.0.0 128.10.35.6 Router 203.16.25.200 203.16.25.0 Figure 8.13 An example of IP network addressing Internet Addressing and Address Resolution longer an IP address broken down into network and host bits according to its class; instead, the route becomes a combination of an address and a mask. The mask indicates how many bits in the address represent the network prefix. For example, the address 200.200.14.20/23 means that the first 23 bits of the binary form of this address represent the network. The bits remaining represent the host. In decimal form, the prefix 23 would look like this: 255.255.254.0. Table 8.1 lists the most commonly used masks, giving their dotted-decimal and prefix values and the number of available host addresses of each type. CIDR defines address assignment and aggregation strategies designed to minimize the size of top-level Internet routing tables, which was another goal of CIDR. The national or regional ISP needs only to advertise its single supernet address, which Table 8.1 CIDR Masking Scheme Mask as Dotted-Decimal Value Mask as Prefix Value Number of Hosts 255.255.255.224 255.255.255.192 255.255.255.128 255.255.255.0 (Class C) 255.255.254.0 255.255.252.0 1,024 255.255.248.0 2,048 255.255.242.0 4,096 255.255.240.0 8,192 255.255.224.0 16,384 255.255.192.0 32,768 255.255.0.0 (Class B) 65,536 255.254.0.0 131,072 255.252.0.0 262,144 255.248.0.0 524,288 Chapter 8 The Internet and IP Infrastructures represents an aggregation of all the subnets within that supernet. Routers in the Internet no longer give any credence to class; routing is entirely based on the CIDR prefix. CIDR does require the use of supporting routing protocols, such as RIPv2, OSPF version 2, Enhanced Interior Gateway Routing Protocol (EIGRP), and BGP4. Subnetting is a term you may have heard in relationship to addressing. It once referred to