257Nortel Business Secure Router 252 Configuration — AdvancedAppendix FIP subnettingIP addressingRouters route based on the network number. The router that delivers the datapacket to the correct destination host uses the host ID.IP classesAn IP address is made up of four octets (eight bits), written in dotted decimalnotation, for example, 192.168.1.1. IP addresses are categorized into differentclasses. The class of an address depends on the value of its first octet.• Class A addresses have a 0 in the left-most bit. In a class A address, the firstoctet is the network number and the remaining three octets make up the hostID.• Class B addresses have a 1 in the left-most bit and a 0 in the next left most bit.In a class B address, the first two octets make up the network number and thetwo remaining octets make up the host ID.• Class C addresses begin (starting from the left) with 1 1 0. In a class Caddress, the first three octets make up the network number and the last octet isthe host ID.• Class D addresses begin with 1 1 1 0. Class D addresses are used formulticasting. (There is also a class “E” address, which is reserved for futureuse.)
