In 1985, RFC 950 defined a standard procedure to support the subnet 
ting, or division, of a single Class A, B, or C network number into
smaller pieces. Subnetting was introduced to overcome some of the
problems that parts of the Internet were beginning to experience with
the classful two level addressing hierarchy, such as:
  Internet routing tables were beginning to grow.
  Local administrators had to request another network number from the
Internet before a new network could be installed at their site. 
Both of these problems were attacked by adding another level of hierar 
chy to the IP addressing structure. Instead of the classful two level hier 
archy, subnetting supports a three level hierarchy. Figure 7 illustrates
the basic idea of subnetting, which is to divide the standard classful
host number field into two parts the subnet number and the host num 
ber on that subnet. 
F I G U R E   7 .   S u b n e t   A d d re s s   H i e r a rc h y
Subnetting attacked the expanding routing table problem by ensuring
that the subnet structure of a network is never visible outside of the
organization's private network. The route from the Internet to any sub 
net of a given IP address is the same, no matter which subnet the desti 
nation host is on. This is because all subnets of a given network number
use the same network prefix but different subnet numbers. The routers
within the private organization need to differentiate between the indi 
vidual subnets, but as far as the Internet routers are concerned, all of
the subnets in the organization are collected into a single routing table
entry. This allows the local administrator to introduce arbitrary com 
plexity into the private network without affecting the size of the Inter 
net's routing tables. 
Subnetting overcame the registered number issue by assigning each
organization one (or at most a few) network numbers from the IPv4
address space. The organization was then free to assign a distinct sub 
network number for each of its internal networks. This allowed the
organization to deploy additional subnets without obtaining a new net 
work number from the Internet. 
U N D E R S TA N D I N G   I P   A D D R E S S I N G






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