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.14IP addresses are split up into four eight-bit numbers called octets for readability.For example,quark.physics.groucho.edu has an IP address of 0x954C0C04, which is written as 149.76.12.4.This format isoften referred to as dotted quad notation.Another reason for this notation is that IP addresses are split into a network number, which is contained in theleading octets, and a host number, which is the remainder.When applying to the NIC for IP addresses, you arenot assigned an address for each single host you plan to use.Instead, you are given a network number and al-lowed to assign all valid IP addresses within this range to hosts on your network according to your preferences.13The version of the Internet Protocol most frequently used on the Internet is Version 4.A lot of effort has been expended in designing areplacement called IP Version 6.IPv6 uses a different addressing scheme and larger addresses.Linux has an implementation of IPv6, butit isn't ready to document it in this book yet.The Linux kernel support for IPv6 is good, but a large number of network applications needto be modified to support it as well.Stay tuned.14Frequently, IP addresses will be assigned to you by the provider from whom you buy your IP connectivity.However, you may alsoapply to the NIC directly for an IP address for your network by sending email to hostmaster@internic.net, or by using the form athttp://www.internic.net/. 35The size of the host part depends on the size of the network.To accommodate different needs, several classes ofnetworks, defining different places to split IP addresses, have been defined.The class networks are describedhere:Class AClass A comprises networks 1.0 through 127.The network number is contained in thefirst octet.This class provides for a 24-bit host part, allowing roughly 1.6 million hosts pernetwork.Class BClass B contains networks 128.0 through 191.255.0; the network number is in the firsttwo octets.This class allows for 16,320 nets with 65,024 hosts each.Class CClass C networks range from 192.0 through 223.255.255.0, with the network number con-tained in the first three octets.This class allows for nearly 2 million networks with up to 254hosts.Classes D, E, and FAddresses falling into the range of 224.0 through 254.0 are either experimental or arereserved for special purpose use and don't specify any network.IP Multicast, which is a servicethat allows material to be transmitted to many points on an internet at one time, has been as-signed addresses from within this range.If we go back to the example in Chapter 1, we find that 149.76.12.4, the address of quark, refers to host 12.4 onthe class B network 149.76.You may have noticed that not all possible values in the previous list were allowed for each octet in the host part.This is because octets 0 and 255 are reserved for special purposes.An address where all host part bits are 0 refersto the network, and an address where all bits of the host part are 1 is called a broadcast address.This refers to allhosts on the specified network simultaneously.Thus, 149.76.255.255 is not a valid host address, but refers to allhosts on network 149.76.A number of network addresses are reserved for special purposes.0 and 127.0 are two such addresses.The first is called the default route, and the latter is the loopback address.The default route has to do with theway the IP routes datagrams.Network 127.0 is reserved for IP traffic local to your host.Usually, address 127.1 will be assigned to aspecial interface on your host, the loopback interface, which acts like a closed circuit.Any IP packet handed tothis interface from TCP or UDP will be returned to them as if it had just arrived from some network.This allowsyou to develop and test networking software without ever using a "real" network.The loopback network alsoallows you to use networking software on a standalone host.This may not be as uncommon as it sounds; forinstance, many UUCP sites don't have IP connectivity at all, but still want to run the INN news system.Forproper operation on Linux, INN requires the loopback interface.Some address ranges from each of the network classes have been set aside and designated "reserved" or "private"address ranges.These addresses are reserved for use by private networks and are not routed on the Internet
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