Cabletron Systems DMS-100 manual Address masks, IP addressing class D

Appendix E: Understanding IP and IP addressing 157

Figure 35 IP addressing: class D

Class D addresses are used to broadcast to all hosts on network.

Base_Tel-15

32 bits

		1110	multicast
	4 bit		28 bits
Class D indicator

Figure 36 IP addressing: class E

Class E addresses are experimental and are generally not used by the IP community.

Base_Tel-16

32 bits

		1111	experimental
	4 bit			28 bits
Class E indicator

Address masks

For administrative or procedural reasons, a network number can be subdivided into subnetworks using a subnetwork mask, also called a subnet or address mask. A network mask is a set of values that masks, or causes the router to ignore, portions of a packet address. This technique allows the administrator to subdivide the networks at levels below the Internet address defined range.

A subnetwork mask identifies to IP the portion of the whole address that identifies the network and subnetwork. Subnetwork masks are represented in decimal values. For example, to indicate that the first two bytes of an address are the network and subnetwork parts and the last two bytes are reserved for hosts, the subnetwork mask is 255.255.0.0.

For instance, assume that IP address 133.101.0.0 has been assigned to a company. Without a subnetwork mask, the last two fields of the address identify individual hosts. In this case, assume the company has five major

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