Cabletron Systems DMS-100 User Manual

Page 157

Appendix E: Understanding IP and IP addressing 157

DMS-100 Family EIU User Guide TELECOM12

Figure 35

IP addressing: class D

Figure 36

IP addressing: class E

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

multicast

bits

4 bit

Class D indicator

1110

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

Base_Tel-15

experimental

bits

4 bit

Class E indicator

1111

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

Base_Tel-16