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136 TCP/IP Overview
EN
Subnets
When an IP address is assigned to an organization, no provision is
made for more than one network being present at that location.
Local network administrators use subnets to partition a network
into several different subnetworks. Splitting a network into subnets
can result in better performance and improved use of limited
network address space.
Subnet Mask
The subnet mask is a mechanism used to divide a single IP network
into several different networks. To subnet the IP address, the
administrator must take part of the host area of the address and
assign it for subnet numbers. The subnet mask “masks” out that
specific portion of the host IP address along with the entire network
section of the address with ones and leaves this portion as zeros,
available to host addressing.
In the table above, “Subnet Masks,” the Class A network 15 has
been assigned to Hewlett-Packard. To allow additional networks at
the HP site, the subnet mask of 255.255.0.0 is used. This address
designates the second byte of the IP address as the subnet address.
Using this designation, each device is uniquely identified on its own
subnet, but HP can incorporate up to 254 subnetworks without
violating their assigned address space.
Table A.3 Subnet Masks
Bytes
(Sections)
Network
Subnet Host Host
Class A
Network
15
xxx xxx xxx
Subnet
Mask
255 255
0 0
Example IP
Address
15
254 64 2