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362 C
HAPTER
16: IP R
OUTING
Subnetwork Portion
The IP address can also contain a subnetwork part at the beginning of the
host part of the IP address. Thus, you can divide a single Class A, B, or C
network internally, allowing the network to appear as a single network to
other external networks. The subnetwork part of the IP address is visible
only to hosts and gateways on the subnetwork.
When an IP address contains a subnetwork part, a subnet mask identifies
the bits that constitute the subnetwork address and the bits that
constitute the host address. A subnet mask is a 32-bit number in the IP
address format. The
1 bits in the subnet mask indicate the network and
subnetwork part of the address. The 0 bits in the subnet mask indicate
the host part of the IP address, as shown in Figure 35.
Figure 35 Subnet Masking
Figure 36 shows an example of an IP address that includes network,
subnetwork, and host parts. Suppose the IP address is
158.101.230.52
with a subnet mask of 255.255.255.0 . Since this is a Class B address, this
address is divided as follows:
■
158.101
is the network part
■
230 is the subnetwork part
■
52 is the host part
As shown in this example, the 32 bits of an IP address and subnet mask
are usually written using an integer shorthand. This notation translates
four consecutive 8-bit groups (octets) into four integers that range from 0
through 255. The subnet mask in the example is written as
255.255.255.0.
IP address Network Subnet and Host
Subnet mask
networ Host subn
Subnet
Network
Apply the subnet mask
Result = subnet/host boundary
Take the IP address
1 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1