tration down, an additional classification of IP addresses is defined. Some host networks may remain rather small (some hundred hosts), others may grow to thousands of hosts. The IP addressing system in IPv4 considers the need for differ- ent sizes of host networks by three major addressing formats. Format A: 127 class-A host networks, 16 772216 hosts per network (IP address 1-2130071432). Intended for national host networks. Format B: 16384 class-B host networks, 65536 hosts per network (IP address 2130071433-3203813256). Intended for regional and large busi- ness host networks. Format C: 524288 class-C host networks, 256 hosts per network (IP address 3203813257-3338030984). Intended for LAN-based host net- works. Figure 3.79 shows the address format for class-A host networks, and the value for host number 113 in network number 12841. The binary value 001 of the format identifier indicates a class-A IP address. Note that IP-addresses are normally writ- ten in dotted decimal notation (which, at least for me, is quite confusing). 3 Logical Network Modelling Format Network number Host number identifier (dec.: 12 841) (dec.: 113) IP address in dotted 000000110001000110001 01110001 decimal notation: 192.098.049. 113 Fig. 3.79 IP address format and notation So far, we have assumed that routing decisions depend only on the (more or less) static structure of how hosts and routers are interconnected. This is a very simplified picture. Routing decisions are equally as dependent on the status of routes (and thereby, of routers). In an open network like IP, there is no central supervisor that surveys the status of all routes of all IP-transport networks and that could inform all routers about problems. Instead, routers are supposed to inform each other and hosts about problems with transporting packages. The messages used for that are defined in a separate protocol, called Internet Control Message Protocol (ICMP), specified in RFC 792. IP process control system ICMP messages handler handler ATM-AAL3/4 source dest. Addr. Add