Variable Length Subnet Mask

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Variable Length of Subnet Mask ( VLSM ) is a subnet mask with variable length. It was first documented in 1985 in RFC 950 and introduced in 1993 as Classless Inter-Domain Routing (CIDR) on the Internet ( RFC 1518 , RFC 1519 , replaced by RFC 4632 in 2006 ).

VLSM is an extended subnetting . This procedure allows a more efficient use of subnets with the background of "wasting" fewer resources. This is realized by assigning unique subnet masks to the previously established subnet ( subnet ).

Multiple, nested division of the address range creates a nested system of address blocks, each of which has a minimum size. This is sufficient in order to be able to supply the respective assigned subnets with a sufficient number of IP addresses .

The advantages are a more effective use of address ranges, the reduction in size of the routing tables by aggregating routes and an increased number of possible subnets.


A subnet is reserved from the address block for addressing point-to-point networks. The subnet, the second etc. are then taken from this for the first point-to-point network.

Web links


  • RFC 950 : Internet Standard Subnetting Procedure (1985)
  • RFC 1518 : An Architecture for IP Address Allocation with CIDR (1993)
  • RFC 1519 : Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy (1993; status: historical)
  • RFC 4632 : Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan