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| 1.5.- IP
subnetting support |
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| Because OPSF attaches an IP address
mask to each advertised route it supports what is known as
IP-subnetting. The address mask indicates the range of addresses being
described by a particular route. For example, a route to 128.185/16
is describing a single route to the range 128.185.0.0 to
128.185.255.255. Similary a route to 128.185.17.36/32 (or simply
128.185.17.36) advertises a route to a single host destination. |
| OSPF also supports what is known as
variable-length subnetting. This means that a single IP class
A, B, or C network number can be broken up into many
subnets of various sizes. |
| OSPF rule for forwarding is: when an
IP packet is forwarded, it is always forwarded to the network that is
the best match for the packet's destination. Here, best match
is synonymous with the longest or most specific match. |
| OSPF also supports IP-supernetting.
This means that many separate networks may be contained in a single address
range, just as a subnetted network is composed of many separate subnets.
This is very important because Area Border Routers summarize the area
contents and distribute this to the backbone, by advertising a single
route for each address range. The cost of this summarized route
is the maximum cost of any of the networks belonging to the specified range. |
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