VLSM (CIDR) Subnet Calculator

VLSM Subnet Calculator

Subnet Name Subnet Size

Please note: This subnetting tool is the best for saving IP space but may produce a /31 subnet mask and a /32 subnet mask, which is not standard practice for subnetting that needs to support multiple hosts.

Understanding the VLSM Subnet Calculator (RFC 3021 Standard)

  • Efficient IP Usage: The VLSM Subnet Calculator follows RFC 3021 to maximize IP address efficiency, especially for point-to-point links.
  • Not for Standard Networks: /31 subnet masks are not typically used for standard network segments that require network and broadcast addresses.

Who Should Use the VLSM Subnet Calculator (RFC 3021 Standard)?

  • Network Administrators: Ideal for those managing point-to-point links who want to conserve IP addresses.
  • Not Recommended for General Networks: Not suitable for networks needing multiple host communications within the same subnet.

How to Use the VLSM Subnet Calculator?

  • Enter Major Network: Input the network address and subnet mask (e.g.,
  • Specify Subnet Count: Indicate the number of subnets needed.
  • Calculate: Click ‘Calculate’ to view the subnetting results.

For more detailed subnetting needs, especially for networks supporting multiple hosts, consider using the Standard/Traditional CIDR (VLSM) Subnet Calculator.

  1. RFC 3021: This RFC, published in 2000, introduced the use of 31-bit subnet masks for IPv4 point-to-point links. This means that a /31 subnet, which provides two IP addresses, can be used efficiently for point-to-point links (like between two routers), whereas traditionally a /30 subnet (providing four addresses, two of which are usable) would have been used. This practice effectively doubles the number of point-to-point links available in an IPv4 network.
  2. Standard Network Practices: Traditionally, /31 subnets were not used in broader network practices, especially in environments that are not point-to-point links. This is because a /31 subnet leaves no room for a network address and a broadcast address, which are typically required in standard network setups.
  3. Vendor Support: Modern network equipment and software generally support /31 subnets for point-to-point links, as per RFC 3021. However, not all network environments or older equipment may support this.
  4. Context of Use: In the context of point-to-point links, using a /31 subnet is perfectly acceptable and is a good practice for conserving IP addresses. However, for general network segments that are not point-to-point, a /31 subnet would not be practical.

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