IP Addressing (IPv4)

In IP version 4, each host on the TCP/IP network has a 32-bit network address—referred to as the IP address—that is unique for each host on the network. If the host will participate on the Internet, this address must also be unique to the Internet. For this reason, Internet IP addresses are controlled by an administrative agency, such as InterNIC.

The IP address is a sequence of four bytes and is written in the form of four decimal integers separated by periods (for example, 0.0.0.0). Each integer is 8 bits long and ranges from 0 to 255. The IP address consists of two parts: a network ID assigned by the InterNIC administrative agency and the host ID assigned by the local administrator. The first integer of the address (0.0.0.0) determines the address type and is referred to as its class. Five classes of IP addresses exist: A, B, C, D, and E. Without going into great detail, the following is a brief description of each class.

Class A Networks

Class A networks are used for very large networks with millions of hosts, such as the Internet. A class A network number uses the first 8 bits of the IP address as its network ID. The remaining 24 bits make up the host part of the IP address. The values assigned to the first byte of class A network numbers fall within the range 0 to 127. For example, consider the IP address 75.4.10.4. The value 75 in the first byte indicates that the host is on a class A network. The remaining bytes, 4.10.4, establish the host address. InterNIC assigns only the first byte of a class A number. Use of the remaining three bytes is left to the discretion of the owner of the network number. Only 126 class A networks can exist because 0 is reserved for the network and 127 is reserved for the loopback device, leaving 001 to 126 as usable addresses. Each of these networks can accommodate up to 16,777,214 hosts.

Class B Networks

Class B networks are medium-size networks, such as universities and large businesses with many hosts. A class B network number uses 16 bits for the network number and 16 bits for host numbers. The first byte of a class B network number is in the range 128 to 191. In the number 129.144.50.56, the first two bytes, 129.144, are assigned by InterNIC and make up the network address. The last two bytes, 50.56, make up the host address and are assigned at the discretion of the network’s owner. A class B network can accommodate a maximum of 65,534 hosts. Again, the first and last addresses on the network are reserved. The 0 host address is reserved for the network, and the 255 address is reserved as the IP broadcast address. Therefore, the actual number of hosts that can be assigned on a class B network is 65,534, not 65,536.

Class C Networks

Class C networks are used for small networks containing fewer than 254 hosts. Class C network numbers use 24 bits for the network number and 8 bits for host numbers. A class C network number occupies the first three bytes of an IP address; only the fourth byte is assigned at the discretion of the network’s owner. The first byte of a class C network number covers the range 192 to 223. The second and third bytes each cover the range 1 to 255. A typical class C address might be 192.5.2.5, with the first three bytes, 192.5.2, forming the network number. The final byte in this example, 5, is the host number. A class C network can accommodate a maximum of 254 hosts out of 256 addresses; again, this is because the first and last values are reserved.

Class D and E Networks

Class D addresses cover the range 224 to 239 and are used for IP multicasting as defined in RFC 988. Class E addresses cover the range 240 to 255 and are reserved for experimental use.

Planning for IP Addressing

The first step in planning for IP addressing on your network is to determine which network class is appropriate for your network. After you have done this, you can obtain the network number from the InterNIC addressing authority. When you receive your network number, you can plan how you will assign the host parts of the IP address. You can reach InterNIC Registration Services in several ways:

• Web site: www.internic.net

  Read the FAQ at this web site to learn more about domain name registration.

• The United States mailing address:

  
  Network Solutions Attn: InterNIC Registration Services 5 
  5 Huntmar Park Dr. 
  Herndon,VA 22070 

• Email: [email protected].

• Phone: 703-742-4777. Phone service is available from 7 a.m. to 7 p.m. (Eastern Standard Time).

• You can also visit the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN is the new nonprofit corporation that is assuming responsibility from the U.S. government for coordinating certain Internet technical functions, including the management of the Internet domain name system. More information about ICANN can be found at www.icann.org.