IP Addressing.  In order for computers to send messages to each other over the internet there needs to be some way for them to locate each other.  The addresses associated with the IP - Internet Protocol are used to locate different machines from routers to computers on the internet.  IP Addressing are 32 bits as shown in the next figure.

 Part of the IP address represents the network, part of it represents the host number for the device on this network. They are grouped into four groups of eight or octets as represented in the following diagram.

 IP addressing defines five address classes: A, B, C, D and E.  Classes D and E are for specialized devices.  Class D is used for multicast groups, Class E is reserved for experimental use.  Classes A, B and C are used more commonly for network addresses.  The majority of networks are numbered using class B and C addresses.

 Class Format Prefix Length Higher Order Bits Address Range Number of Network Addresses A N.H.H.H 8 bits 0 1.0.0.0 to 126.0.0.0 256 B N.N.H.H 16 bits 10 128.0.0.0 to 191.255.0.0 (256)2 = 65,563 C N.N.N.H 34 bits 110 192.0.0.0 to 223.255.255.0 (256)3 = 16,777,216

 What the table attempts to imply is that Class A addresses are network and host portions are configured as illustrated in the following.

 For class B the addresses are likely to look like.

 For class C the addresses are of the following form.

 So if you think about the proliferation of computers and LANs on the internet there is the very basic problem of just locating the LANs and all of their machines that are internet capable.  From the classes of addresses how many networks you can locate and how many machines.

 Class Number of Network Addresses Number of Computer Addresses A 256 (256)3 = 16,777,216 B (256)2 = 65,563 (256)2 = 65,563 C (256)3 = 16,777,216 256

 For class A you could not have hardly any LANs on the network.  For class B you can have a few LANs, but nowhere near enough to give a fully functioning internet.  But even if you were satisfied with just locating/connecting around 65,000 different LANs, you wouldn't be happy with having IP addresses for only another 65,000 machines.  Finally, for class C addresses you could at least locate a healthy 16 million LANs, but this leaves only a very small number for other machines.What these classes imply is that with a 32 bit addressing scheme you are going to need to use unique IP addresses for locating networks and then somehow use some sort of local addressing scheme to locate each of the machines on each LAN.  This is the issue that this and the immediately following pages address.