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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnet Addressing ("Subnetting") Concepts

IP Subnet Addressing ("Subnetting") Concepts 1 2 3 IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing

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.

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 system:

• 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.

IP Subnet Addressing ("Subnetting") Concepts 1 2 3 IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing