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Church Of The
Swimming Elephant

A Simple Subnet Connected: An Internet Encyclopedia
A Simple Subnet

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Up: Programmed Instruction Course
Up: Subnetting and CIDR
Prev: Fundamental Principle of Routing
Next: Reserved Subnet Numbers

A Simple Subnet Consider a group of hosts attached to an Ethernet segment. A single IP address prefix would be assigned to the entire Ethernet, in this example Each device attached to the Ethernet would have a unique IP address matching the prefix. The Ethernet, its address prefix, and its hosts would be collectively refered to as a subnet. An address such as, which doesn't match the Ethernet's prefix, would not be a valid IP address on this subnet.

The router on the left would maintain a table listing the Ethernet addresses for every device on this subnet, and therefore be capable of delivering packets directly to these hosts. Routers elsewhere in the network would only maintain a single entry for the entire subnet, indicating that traffic for should be delivered to this router.

An Ethernet with a larger number of hosts would require a shorter prefix (matching more addresses), while an Ethernet with only a few hosts could be assigned a longer prefix.

As a more complex example, consider a group of closely interconnected Ethernets, each one a subnet. Then there might be a single address prefix for the entire group. The group's prefix would contain all the address prefixes for the individual subnets.

Next: Reserved Subnet Numbers

Connected: An Internet Encyclopedia
A Simple Subnet


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