B-2 User’s Reference GuideIP addresses indicate both the identity of the network and the identity of the individual host on the network. Thenumber of bits used for the network number and the number of bits used for the host number can vary, as longas certain rules are followed. The local network manager assigns IP host numbers to individual machines.IP addresses are maintained and assigned by the InterNIC, a quasi-governmental organization now increasinglyunder the auspices of private industry.Note: It’s very common for an organization to obtain an IP address from a third party, usually an Internetservice provider (ISP). ISPs usually issue an IP address when they are contracted to provide Internet accessservices.The InterNIC (the NIC stands for Network Information Center) divides IP addresses into several classes.Classes A, B, and C are assigned to organizations that request addresses. In Class A networks, the first byte ofan IP address is reserved for the network portion of the address. Class B networks reserve the first two bytesof an IP address for the network address. Class C networks reserve the first three bytes of an IP address for thenetwork address. In all cases, a network manager can decide to use subnetting to assign even more bits to thenetwork portion of the IP address, but never less than the class requires. The following section gives moreinformation on subnetting.Class A networks have a small number of possible network numbers, but a large number of possible hostnumbers. Conversely, Class C networks have a small number of possible host numbers, but a large number ofpossible network numbers. Thus, the InterNIC assigns Class A addresses to large organizations that have verylarge numbers of IP hosts, while smaller organizations, with fewer hosts, get Class B or Class C addresses. Youcan tell the various classes apart by the value of the first (or high-order) byte. Class A networks use values from1 to 127, Class B networks use values from 128 to 191, and Class C networks use values from 192 to 223.The following table summarizes some of the differences between Class A, B, and C networks.Subnets and subnet masksOften an entire organization is assigned only one IP network number. If the organization has several IP networksconnected together with IP routers, the network manager can use subnetting to distinguish between thesenetworks, even though they all use the same network number. Each physical network becomes a subnet with aunique subnet number.Subnet numbers appear within IP addresses, along with network numbers and host numbers. Since an IPaddress is always 32 bits long, using subnet numbers means either the network number or the host numbersmust use fewer bits in order to leave room for the subnet numbers. Since the InterNIC assigns the networknumber proper, it should not change, so the subnet numbers must be created out of bits that would otherwisebe part of the host numbers.Class First byteNumber ofnetworkspossible perclassNumber ofhostspossible pernetworkFormat of address(without subnetting) ExampleA 1–127 127 16,777,214 net.host.host.host 97.3.14.250B 128–191 16,384 65,534 net.net.host.host 140.100.10.11C 192–223 2,097,152 254 net.net.net.host 197.204.13.7
