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A short introduction to IP Addresses
In order for systems to locate each other in a distributed environment, nodes are given explicit addresses that uniquely identify the particular network the system is on and uniquely identify the system to that particular network. When these two identifiers are combined, the result is a globally-unique address.
This address, known as ?IP address?, as ?IP number?, or merely as ?IP? is a code made up of numbers separated by three dots that identifies a particular computer on the Internet. These addresses are actually 32-bit binary numbers, consisting of the two subaddresses (identifiers) mentioned above which, respectively, identify the network and the host to the network, with an imaginary boundary separating the two. An IP address is, as such, generally shown as 4 octets of numbers from 0-255 represented in decimal form instead of binary form.
For example, the address 18.104.22.168 represents the 32-bit binary number 10101000.11010100.11100010.11001100.
The binary number is important because that will determine which class of network the IP address belongs to. The Class of the address determines which part belongs to the network address and which part belongs to the node address (see IP address Classes further on).
The location of the boundary between the network and host portions of an IP address is determined through the use of a subnet mask. This is another 32-bit binary number which acts like a filter when it is applied to the 32-bit IP address. By comparing a subnet mask with an IP address, systems can determine which portion of the IP address relates to the network and which portion relates to the host. Anywhere the subnet mask has a bit set to ?1?, the underlying bit in the IP address is part of the network address. Anywhere the subnet mask is set to ?0?, the related bit in the IP address is part of the host address.
The size of a network is a function of the number of bits used to identify the host portion of the address. If a subnet mask shows that 8 bits are used for the host portion of the address block, a maximum of 256 host addresses are available for that specific network. If a subnet mask shows that 16 bits are used for the host portion of the address block, a maximum of 65,536 possible host addresses are available for use on that network.
An Internet Service Provider (ISP) will generally assign either a static IP address (always the same) or a dynamic address (changes every time one logs on). ISPs and organizations usually apply to the InterNIC for a range of IP addresses so that all clients have similar addresses. There are about 4.3 billion IP addresses. The class-based, legacy addressing scheme places heavy restrictions on the distribution of these addresses. TCP/IP networks are inherently router-based, and it takes much less overhead to keep track of a few networks than millions of them.
Class A addresses always have the first bit of their IP addresses set to ?0?. Since Class A networks have an 8-bit network mask, the use of a leading zero leaves only 7 bits for the network portion of the address, allowing for a maximum of 128 possible network numbers, ranging from 0.0.0.0 ? 127.0.0.0. Number 127.x.x.x is reserved for loopback, used for internal testing on the local machine.
Class B addresses always have the first bit set to ?1? and their second bit set to ?0?. Since Class B addresses have a 16-bit network mask, the use of a leading ?10? bit-pattern leaves 14 bits for the network portion of the address, allowing for a maximum of 16,384 networks, ranging from 22.214.171.124 ? 126.96.36.199.
Class C addresses have their first two bits set to ?1? and their third bit set to ?0?. Since Class C addresses have a 24-bit network mask, this leaves 21 bits for the network portion of the address, allowing for a maximum of 2,097,152 network addresses, ranging from 192.0.0.0 ? 188.8.131.52.
Class D addresses are used for multicasting applications. Class D addresses have their first three bits set to ?1? and their fourth bit set to ?0?. Class D addresses are 32-bit network addresses, meaning that all the values within the range of 184.108.40.206 ? 220.127.116.11 are used to uniquely identify multicast groups. There are no host addresses within the Class D address space, since all the hosts within a group share the group?s IP address for receiver purposes.
Class E addresses are defined as experimental and are reserved for future testing purposes. They have never been documented or utilized in a standard way.