Installing and Administering LAN/9000 Software

144 Chapter6
Network Addressing
Subnet Addresses
A or Class B network.
Assigning a Fixed-Length Subnet Mask
Subnet addressing is implemented by specifying a 32-bit subnet mask in
the ifconfig command or the netconf file when a LAN interface card
is assigned an internet address. All nodes on a subnet must have the
same subnet mask. Under the fixed-length subnet address scheme, all
subnets on a network, must have the same subnet mask.
The subnet mask is ANDed with the address attached to a message
coming across the network to determine if that message should be routed
to a node on the local network or ignored.
Before a subnet mask is applied, the first three octets of a Class C IP
address represent the network address. If the subnet mask
255.255.255.224 is applied, the first three octets of the address and the
first three bits of the last octet indicate the subnet address while the last
five bits of the last octet indicate the host address. This is shown in the
following figure.
Figure 6-7 Fixed-Length Subnet Mask Example
With this subnet mask, up to 8 subnets with 30 nodes per subnet are
possible. Table 6-5 below lists the valid internet address ranges for nodes
in a network with the subnet mask 255.255.255.224.
11 111 11 111 11 111 1 1111 110011111 000
Class C Subnet Mask:
255 . 255 . 255 . 224
network address subnet portion of
host address
host portion of
host address