Reference Manual for the Model RP614 Web Safe Router
Network and Routing Basics B-5
Subnet Addressing
By looking at the addressingstructures, you can see that even with a Class C address, there are a
largenumber of hosts per network. Such a structure is an inefficient useof addresses if each end of
arouted link requires a different network number. It is unlikely that the smalleroffice LANs would
have thatmany devices. You can resolve this problem by usinga technique known as subnet
addressing.
Subnet addressingallows us to split one IP network address into smaller multiple physical
networksknown as subnetworks. Some of the node numbers are used as a subnet number instead.
A ClassB address gives us 16 bits of node numbers translating to 64,000 nodes. Most
organizationsdo not use 64,000 nodes, so there are free bits that can be reassigned.Subnet
addressingmakes use of those bits that are free, as shown below.
Figure B-2. Example of Subnetting a Class B Address
A Class B addresscan be effectivelytranslated into multiple Class C addresses. For example, the
IP address of 172.16.0.0is assigned,but node addresses are limited to 255 maximum, allowing
eightextra bits to use as a subnet address. The IP address of 172.16.97.235 would be interpretedas
IP networkaddress 172.16, subnet number 97, and node number 235. In addition to extending
the number of addressesavailable, subnet addressing provides other benefits. Subnet addressing
allows a networkmanager to construct an address scheme for the network by using different
subnetsfor other geographical locations in the network or for other departments in the
organization.
Although thepreceding example uses the entire third octet for a subnet address, notethat you are
not restrictedto octet boundaries in subnetting. To create more network numbers, you need only
shiftsome 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 one 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 192.68.135.0with hosts 192.68.135.1 to 129.68.135.126, and the second subnet has
network number192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254.
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