Subnet Addressing - NETGEAR PS101 Reference Manual

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Subnet Addressing

By looking at the addressing structures, you can see that even with a Class C address there are a large
number of hosts per network. Such a structure is an inefficient use of addresses if each end of a routed
link requires a different network number. It is unlikely that the smaller office LANs would have that
many devices. You can resolve this problem by using a technique known as subnet addressing.
Subnet addressing allows us to split one IP network address into smaller multiple physical networks
known as subnetworks. Some of the node numbers are used as a subnet number instead. A Class B
address gives us 16 bits of node numbers translating to 64,000 nodes. Most organizations do not use
64,000 nodes, so there are free bits that can be reassigned. Subnet addressing makes use of those bits
that are free, as illustrated below.
Example of Subnetting a Class B Address
A Class B address can be effectively translated into multiple Class C addresses. For example, the IP
address of 172.16.0.0 is assigned, but node addresses are limited to 255 maximum, allowing 8 extra
bits to use as a subnet address. The IP address of 172.16.97.235 would be interpreted as IP network
address 172.16, subnet number 97, and node number 235. In addition to extending the number of
addresses available, subnet addressing provides other benefits. Subnet addressing allows a network
manager to construct an address scheme for the network by using different subnets for other
geographical locations in the network or for other departments in the organization.
Although the preceding example uses the entire third octet for a subnet address, note that you are not
restricted to octet boundaries in subnetting. To create more network numbers, you need only shift some
bits from the host address to the network address. For instance, to partition a Class C network number
(192.68.135.0) into two, you shift 1 bit from the host address to the network address. The new netmask
(or subnet mask) is 255.255.255.128. The first subnet has network number 192.68.135.0 with hosts
192.68.135.1 to 129.68.135.126, and the second subnet has network number 192.68.135.128 with hosts
192.68.135.129 to 192.68.135.254.
The table below lists the additional subnet mask bits in dotted-decimal notation. To use the table, write
down the original class netmask and replace the 0 value octets with the dotted-decimal value of the
additional subnet bits. For instance, to partition your Class C network 204.247.203.0 with subnet mask
255.255.255.0 into 16 subnets (4 bits), the new subnet mask becomes 255.255.255.240.
Netmask Notation Translation Table for One Octet
Number of Bits Dotted-Decimal Value
1 128
2 192
3 224
4 240
5 248
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The number 192.68.135.127 is not assigned because it is the broadcast address of the
first subnet. And 192.68.135.128 is not assigned because it is the network address of
the second subnet.
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