VLSM:
Variable Length Subnet Mask is a
technique that network administrators employ in order to use their IP
subnet(s) in a more effective manner. By using VLSM, a long mask can be
used on a network that has a few hosts and a short net mask on subnets
that have a large number of hosts. To use VLSM, however, a routing
protocol that supports it has to be used. Cisco routers support the
concept with the following protocols: Integrated IS-IS (Integrated
Intermediate System to Intermediate System), EIGRP (Enhanced Interior
Gateway Routing Protocol), RIP v2, Open Shortest Path First (OSPF), and
static routing. VLSM also allows more than one subnet mask within the
same network address space, which is also referred to as “subnetting a
subnet.
How VLSM works:
Historically, routing protocols required
that a single network use the same subnet mask. VLSM allows networks to
have different subnet masks if the routing protocol on the network on
which it is employed supports it. VLSM also breaks convention in that it
uses the first and last subnets, which were traditionally reserved to
alleviate the confusion caused when the network and subnet had the same
address. When this is done, VLSM supports eight usable subnets that can
each support 30 hosts.
Where is VLSM used:
VLSM is often used in a college campus
environment. If the network administrator has a Class B block of
addresses to use on several campuses, he/she normally uses variable
length subnets. The subnets may then be further divided by building and
workgroup on the campuses, which would require different numbers of
addresses. If fixed subnet masks were used to allocate the same number
of IP addresses to the locations, a number of addresses would be wasted.
If VLSM is employed, then there is less waste in the allocated address
space across all of the campus locations, giving more room for network
growth.
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