IP ADDRESSING

12/12/97

Internet Protocol (IP) addresses: IP addresses are 32 bit network addresses usually expressed as four decimal digits in the range 0-255 separated by periods. The addressing was devised for the internet but is used by all devices using the increasingly ubiquitous TCP/IP network protocol. In concept, all devices should have unique addresses. In practice, all devices that can see each other must have unique addresses. The usual practice is to define either a Class C subnet with up to 255 unique addresses or a Class B subnet with 65535 addresses. In many cases an address that is present in the local network can not easily be differentiated from an address that is present on an external network such as the Internet although a few addresses are reserved for local use.

Although 32 bits can accommodate over a thousand million hosts, the allocation of addresses is very inefficient. Many addresses are not easily allocatable to real hosts. A 48 bit network addressing scheme referred to as IPv6 has been defined to provide more satisfactory addressing.

The following IP addresses are allocated for local machines and will never be used by an external network

        10.0.0.0        -   10.255.255.255
        172.16.0.0      -   172.31.255.255
        192.168.0.0     -   192.168.255.255

127.0.0.1 is always the local machine.

The broadcast address is a special address that every host on the network listens to in addition to its own unique address. This is usually the highest legal value within range, but some use the network's external address as the internal broadcast address. Either is valid, but all computers on the network must use the same broadcast addressing.

Addresses starting with 0 thru 127 are called class A addresses. Only the first numeric value (7 usable bits) is used in Internet routing. Class B addresses start with 128-191 and are routed on the basis of the first two numbers -- basically 14 bits of addressing. Class C networks start with 192-223 and use 3 numbers -- 21 usable bits. Class D addresses starting with 224-239 are reserved for multicast. The remaining 16 address groups are reserved for future use.

Rewritten Nov 2008