Subnetting hides the internal network organization to external routers and
thus simplies routing. For instance, a subnetted Class B address would require
fewer routes than the equivalent number of Class C addresses. Shorter routing
tables mean faster network transfers.
Subnetting allows address administration to be decentralized. Besides
technical advantages, this approach may also provide political benefits for the
organization. For instance, an administrator could assi gn a subnet to a
department, which would then be responsible for their own network management.
Subnetting can help overcome distance limitations of physical networks by
dividing up a physical network into individually addressed networks so they can
be connected logically with routers.
Example: Subnetting a Class C Network
One of the first things a network administrator needs to do is define the
requirements for the network. The logical place to start is to consider how many
hosts are on the network.
Using the maximum number of hosts on one Ethernet segment is generally not
good practice because it could create performance problems due to network
congestion. If you only have one Class C address assigned to your network then
what can you do? Refer to our table above that depicts the Class C address
subnetting network number-host trade off.
Even though a Class C address can support up to 254 hosts, in my experience,
60-80 hosts is a good number for most LANs using of fice automation tools. I've
seen overloaded Ethernet segments--with over 100 hosts--at client sites. My
recommendation is that they segment their LAN in half or even further. Also,
many hub cards come with 24 ports per card, which makes it easy to segment in
24-host multiples provided that the hub supports multiple segments on the
backplane. Many do.
One reasonable approach would be to select six subnets each with 30 hosts.
Although two subnets with 62 hosts is also feasible, it is not as flexible
because there are only two subnets. The other alternatives that use more subnets
probably don't provide enough hosts per subnet.
Subnets 0 and 7 are unusable because they are used for special addressing
situations. For instance, a subnet of 7 (all one bits) is reserved for an all
subnets-directed broadcast (a broadcast sent to all subnets of the specified
subnetted network) when the host bits are all one. This leaves subnets 1 through
6 available for use.
In each subnet, the first ho st number (0) is reserved, and the resulting
number is known as the network number. The last number in each subnet is
reserved for the broadcast address, and cannot be used for a host address.
Consequently, in this case there are only 30 host addresses available for each
subnet.
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