A binary number that uniquely identifies a host connected to the Internet and running the IPv4 protocol suite. The IP packet header provides an originating address field so that a host can identify itself as the originator of a packet.The IP packet header also provides a destination address field so that an originating host can identity the target host for which a packet is intended. Based on that information, the Internet routers can act to deliver the packet to the target host, which can respond to the originating host, as appropriate. The IPv4 address field size is 32 bits, which yields the theoretical potential for 2 32 or 4,294,967,296 unique addresses. All IP addresses are written in dotted decimal notation, which also is known as dotted quad format in the case of IPv4. Each IPv4 address field is divided into four fields separated by dots and expressed as xxx.xxx.xxx.xxx with each field given a value in decimal notation of 0­255, the range expressed by a single octet in binary notation (2 8 = 256, or 0­255). (Note: Leading zeros are suppressed.) The addressing architecture defines five address formats, each of which begins with one, two, three, or four bits that identify the class of the network (Class A, B, C, D, or E).The Network ID space identifies the specific network, and the Host ID space identifies the specific host computers on the network. Classes A, B, and C are each associated with a range of IP addresses that supports a limited number of networks per class and a limited number of hosts per network, as detailed in Table I-3. Class D addresses are reserved for multicast purposes and Class E addresses are reserved for future use.
Table I-3: IPv4 Address Classes

ClassAddress RangeNetworks per ClassHosts per NetworkA1.0.0.1 to 126.255.255.25412616,777,214B1 28.0.0.1 to 191.255.255.25416,38465,534C19 2.0.0.1 to 223.255.255.2542,097,152254DRe served for multicast purposesEReserved for experimental useThe Internet Corporation for Assigned Names and Numbers (ICANN) assigns public IP addresses to organizations desiring to place computers on the Internet.Theoretically speaking, the size of the user organ ization determines the IP Class and, therefore, the number of available host addresses, although no more Class A addresses are being assigned and virtually all Class B addresses have already been assigned.The user organization assigns the host address numbers internally and can reassign them on the basis of either static or dynamic addressing. Static addresses are permanently or semi-permanently associated with a specific host. Dynamic addresses are assigned each time a connection is established.There are both public and private IP address spaces. Computers on private LANs running the TCP/IP protocol suite do not require public addresses, at least not for internal use within the LAN domain. Private IP addresses are set aside for such purposes and will never be used publicly.The Internet Protocol version 6 (IPv6) addressing scheme is quite different.

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