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Table Of Contents  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

Previous Topic/Section
IP Subnet Addressing ("Subnetting") Concepts
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing
Next Topic/Section

IP Subnet Addressing Overview, Motivation, and Advantages
(Page 3 of 3)

The Impact of Subnetting on Addressing and Routing

The change to subnetting affects both addressing and routing in IP networks. Addressing changes of course, because instead of having just a network ID and host ID, we now also have a subnet ID to be concerned with. The size of the subnet ID can vary for each network, so an additional piece of information is needed to supplement the IP address to indicate what part of the address is the subnet ID and what part is the host ID. This is a 32-bit number commonly called a subnet mask. The mask is used both for calculating subnet and host addresses, and by routers for determining how to move IP datagrams around a subnetted network.

Routing changes because of the additional level of hierarchy. In regular “classful” addressing, when a router receives an IP datagram, it only needs to decide if the destination is on the same network or a different network. Under subnetting, it must also look at the subnet ID of the destination and make one of three choices: same subnet, different subnet on the same network, or different network. Again, this is done using the subnet mask. Changes are also required to routing protocols, such as the Routing Information Protocol (RIP), to deal with subnets and subnet masks.

Key Concept: Subnet addressing adds an additional hierarchical level to how IP addresses are interpreted, by dividing an organization’s IP network into subnets. This allows each organization to structure its address space to match its internal physical networks, rather than being forced to treat them a flat block. This solves a number of problems with the original “classful” addressing scheme, but requires changes to how addressing and routing work, as well as modifications to several TCP/IP protocols.


It’s funny, but the main drawbacks to subnetting, compared with the older addressing scheme, have more to do with understanding how subnetting works than the technology itself! More effort is required to deal with addressing and routing in a subnet environment, and administrators must learn how to subdivide their network into subnets and properly assign addresses. This can be a bit confusing to someone who is new to subnetting. However, the technology today is quite well-established so even this is not much of a problem. For the newcomer, having a handy reference guide like this one certainly helps. J


Previous Topic/Section
IP Subnet Addressing ("Subnetting") Concepts
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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