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54b. IP Address Classifications

There is a formal structure to the assignment of IP addresses. IP addresses are assigned by the Network Information Center (NIC) who is a central authority with the responsibility of assigning network addresses.

There are several classifications of IP addresses. They include network addresses and special purpose addresses.

Class A addresses

	IP address range 		1.0.0.0 to 127.0.0.0

	Number of networks available:	125 (see special addresses below)

	Number of hosts per network:	16,777,214

	Net Mask:			255.0.0.0  (first 8 bits are ones)

	Special Addresses:		10.0.0.0 is used for networks not connected to the Internet

					127.0.0.0 is the loopback address for testing (see ping)

Class A addresses always have bit 0 set to 0, bits 1-7 are used as the network ID. Bits 8-31 are used as the host ID.

Class A networks are used by very large companies such as IBM, US Dept of Defense and AT&T. Appendix E: IP Protocol Address Space lists the IP addresses and the organizations that use them.

Class B addresses

	IP address range 		128.0.0.0 to 191.0.0.0

	Number of networks available:	16,382 (see special addresses below)

	Number of hosts per network:	65,534

	Net Mask:			255.255.0.0	(first 16 bits are ones)

	Special Addresses:		172.16.0.0 to 172.31.0.0 are used for networks not

					connected to the Internet

Class B addresses always have bit 0 and 1 set to 10, bits 2-15 are used as the network ID. Bits 16-31 are used as the host ID. Class B networks are assigned to large companies and universities.

Class C addresses

	IP address range 		192.0.0.0 to 223.0.0.0

	Number of networks available:	2,097,150 (see special addresses below)

	Number of hosts per network:	254

	Net Mask:			255.255.255.0	(first 24 bits are ones)

	Special Addresses:		192.168.1.0 to 192.168.255.0 are used for networks not

					connected to the Internet

Class C addresses always have bits 0-2 set to 110, bits 3-24 are used as the network ID. Bits 25-31 are used as the host ID. Class C network addresses are assigned to small companies and local Internet providers.

Class D Addresses

	IP address range 		224.0.0.0 to 239.0.0.0

	Use:				Multicasting addresses

Class D addresses always have bits 0-3 set to 1110, bits 4-31 are used as the Multicast address.

Class D network addresses are used by multicasting. Multicasting is a method of reducing network traffic. Rather than send a separate datagram to each host if multiple host require the same information. A special multicast address can be used where one datagram is read by many hosts. Appendix F: IP Multicast Addresses lists the assigned IP multicast address space.

Class E Addresses

	IP address range 		240.0.0.0 to 255.0.0.0

	Use:				Reserved by the Internet for its own use.

If you try to ping a Class E address, you should get the error message that says that it is an invalid IP address.

Reserved IP Addresses

The following IP addresses are reserved:

	127.0.0.0		Network addresses used for localhost mode (testing IP stack)

	255.255.255.255		An IP address consisting of all 1s in binary (255). Broadcast address

	x.x.x.0			An IP address with the host portion consisting of 0s. Used to indicate

				the network address. Newer routers have the option of allowing these

				addresses.

	224.0.0.0 - 255.0.0.0	Class D addresses.

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