User Guide

32 NetWare TCP/IP Administration Guide
NetWare TCP/IP Administration Guide
103-000151-001
August 30, 2001
Novell Confidential
Manual 99a38 July 17, 2001
Subnet Zero
Subnet zero is a subnet with all the bits in the subnet field of the IP address set
to 0. For example, subnet 130.57.0.0, with a mask of 255.255.240.0, is a
subnet zero of network 130.57, as shown in Figure 13 on page 32.
Figure 13 A subnet zero
The official IP specification reserves the subnet addresses with all zeros and
all ones and does not allow them to be used as subnet addresses. However, this
policy wastes one subnet in the IP address space. To counteract this limitation,
Novell's TCP/IP implementation enables the use of subnet zero.
Variable Size Subnets
The subnets of a network can have different length subnet masks, called
variable length subnet masks. These subnets are called variable because the
size, or length, of the subnet varies from subnet to subnet.
A subnet mask defines the number of bits that can be used to define the subnet
and the number of bits to define the host. As the subnet mask increases, the
number of hosts on a subnet decreases. As the subnet mask decreases, the
number of hosts that can be defined increases.
Some network configurations have individual subnets with a large number of
hosts and other subnets with a small number of hosts. Using the same subnet
masks on all subnets can mean either of the following:
 The mask is too small and you do not have enough subnet numbers for all
your subnets.
 The mask is too big and you do not have enough host IDs for all your
hosts on a subnet.
If the mask is too small or too big, use a variable size subnet. By varying the
size of the subnet mask used on a network, you can match the number and size
of subnets to your configuration.
Subnet
Mask:
130.57.0.1:
Subnet Address
Network Address
Local Host Portion
10000010.00111001.00000000.00000001 IP Address on Subnet 0
11111111.11111111.11110000.00000000