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IP Subnet Addressing Overview, Motivation, and Advantages
(Page 2 of 3)
The Development of Subnet Addressing
In order to address this problem
adequately, an enhancement was required to the classful
addressing scheme. This change was outlined in RFC 950, which defined
a new addressing procedure called subnet addressing or subnetting.
(This RFC was published way back in 1985, which surprises some people!)
The basic idea behind subnet addressing
is to add an additional hierarchical level in the way IP addresses are
interpreted. The concept of a network remains unchanged, but instead
of having just hosts within a network, a new two-level hierarchy
is created: subnets and hosts. Each subnet is a subnetwork, and
functions much the way a full network does in conventional classful
addressing. A three-level hierarchy is thus created: networks, which
contain subnets, each of which then has a number of hosts.
Thus, instead of an organization
having to lump all of its hosts under that network in an unstructured
manner, it can organize hosts into subnets that reflect the way internal
networks are structured. These subnets fit within the network identifier
assigned to the organization, just as all the unorganized
hosts used to.
Advantages of Subnet Addressing
In essence, subnet addressing allows
each organization to have its own internet within the Internet.
Just as the real Internet looks only at networks and hosts, a two-level
hierarchy, each organization can now also have subnets and hosts within
their network. This change provides numerous advantages over the old
- Better Match to Physical Network Structure:
Hosts can be grouped into subnets that reflect the way they are actually
structured in the organization's physical network.
- Flexibility: The number of subnets and
number of hosts per subnet can be customized for each organization.
Each can decide on its own subnet structure and change it as required.
- Invisibility To Public Internet: Subnetting
was implemented so that the internal division of a network into subnets
is visible only within the organization; to the rest of the Internet
the organization is still just one big, flat, network. This
also means that any changes made to the internal structure are not visible
outside the organization.
- No Need To Request New IP Addresses: Organizations
don't have to constantly requisition more IP addresses, as they would
in the workaround of using multiple small Class C blocks.
- No Routing Table Entry Proliferation:
Since the subnet structure exists only within the organization, routers
outside that organization know nothing about it. The organization still
maintains a single (or perhaps a few) routing table entries for all
of its devices. Only routers inside the organization need to worry about
routing between subnets.
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Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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