5-8
APX 1000™ Getting Started Guide
Configuring the Shelf Controller
Providing a basic system IP configuration
Netmasks
If no netmask is specified, the unit uses a default mask based on the class of the IP address that
is supplied. Table 5-1 shows address classes and the number of network bits in the default
mask for each class.
For example, a class C address, such as 198.5.248.40, has 24 network bits, leaving 8 bits for
the host portion of the address. If no subnet mask is specified for a class C address, the
APX 1000 unit uses the default mask of 24 bits, as shown in Figure 5-4.
Figure 5-4. Default netmask for class C IP address
By default, this address is displayed as 198.5.248.40/24
Subnets
Subnets are permitted under the CIDR syntax. A subnet address specifies a number of network
bits that does not adhere to the Class A, B, or C network limits. For example, the following
address specifies a 29-bit subnet:
ip-address = 198.5.248.40/29
In this address, 29 bits of the address are used to specify the network. The three remaining bits
are used to specify eight addresses on the subnet. With three bits used to specify hosts on a
29-bit subnet, eight different bit combinations are possible. Of those eight possible host
addresses, two are reserved:
000—Reserved for the network (base address)
001
010
100
110
101
011
111—Reserved for the broadcast address of the subnet
!
Caution:
To avoid routing problems, make sure that you treat zero subnets consistently
throughout your network. Early implementations of TCP/IP did not allow zero subnets
(subnets with the same base address as a class A, B, or C network). For example, the subnet
Table 5-1. IP address classes and number of network bits
Class
Address range
Default network bits
Class A
0.0.0.0 through 127.255.255.255
8
Class B
128.0.0.0 through 191.255.255.255
16
Class C
192.0.0.0 through 223.255.255.255
24
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
Default 24 bits
