Network Numbering Example; Figure 37 Address Mask Example - Nortel DMS-100 User Manual

158 Appendix E: Understanding IP and IP addressing
departments. Each department expects to use fewer than 254 host addresses,
so the entire third byte of the address is chosen for the subnetwork number. To
reserve the third byte for the subnetwork number, they use subnetwork mask
255.255.255.0. Figure 37 on page 158 illustrates this point.

Figure 37 Address mask example

There is no official requirement that the subnet mask consist only of
contiguous bits. However, in the presence of variable width subnetworks, non-
contiguous masks can lead to ambiguous routing when subnet masks partially
overlap (such as 255.255.255.0 and 255.255.0.255).

Network numbering example

Figure 38 on page 159 illustrates an example of a simple network numbering
scheme for an organization that performs all networking internally. Since, for
security reasons, they never expect to attach to the outside world, they use their
own set of network numbers. The network has two routers: one for the
Corporate Networking group to interconnect non-engineering users, and a
second for a large, computer-intensive department, such as Engineering.
297-8991-910 Standard 03.01 August 1999
Network node (133.101.1.8)
Class B
network ID
133 101
+ Subnet mask (255.255.255.0)
Class B network ID
255 255
= Subnet network (for example, 133.101.1.0)
Class B net
133
101
Network 133.101.1.0 is a subnet of network 131.101.0.0.
Subnet ID Host ID
8
1
Subnet ID Host ID
255 0
Subnet ID Host ID
1
0