VLSM Allocation Calculator

About this interactive

Fixed-length subnetting gives every subnet the same size, so a two-host router link gets as many addresses as the biggest LAN. Variable-length subnet masking (VLSM) sizes each subnet to its own network. The method is the same every time. Sort the networks from most hosts to fewest. Give each the smallest block whose usable addresses hold it: the block size less two, for the network and broadcast addresses. Then lay the blocks end to end from the start of the space. Take 192.168.12.0/24 with LANs of 70 and 20 hosts and one router link. 70 hosts need 72 addresses, so a /25: 192.168.12.0 to .127. 20 hosts fit a /27, 32 addresses with 30 usable, which starts at .128 and ends at .159. The link needs two usable addresses, a /30, from .160 to .163. Largest first matters because a block can only start on a multiple of its own size; put the small blocks first and the big ones have to skip ahead, stranding the addresses in between. Nothing overlaps, and .164 to .255 are free for later. Type addresses in dotted decimal and counts in digits. Anything you miss shows the answer and how to work it out. Seven of the twenty-five questions are drawn each run.

What you'll learn

Allocate a /24 to networks of different sizes with VLSM, largest first, giving each the smallest block that holds its hosts
Calculate the prefix, mask, network address, usable range and broadcast address of each network in a VLSM plan, and the addresses left free
Compare a VLSM plan with one mask for every subnet, by the subnets it gives and the addresses it takes

Aligned to

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing.

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

Cisco CCST Networking

2.2 Identify IPv4 addresses and subnet formats

Key terms

Variable Length Subnet Mask: A subnetting technique that allows a network administrator to divide an address space into subnets of different sizes by using different prefix lengths, allocating address space more efficiently than fixed-length subnetting. VLSM requires a classless routing protocol such as OSPF, EIGRP, or RIPv2 that carries prefix length information in routing updates.
Fixed-Length Subnet Masking: A subnetting method in which all subnets within a network use the same prefix length, resulting in equal-sized address blocks regardless of actual host requirements.
Network Address: The first address in an IP network range that identifies the network itself and cannot be assigned to a host. It is formed when all host bits are set to zero.
Broadcast Address: The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
Usable Host Range: The set of IP addresses within a subnet that can actually be assigned to network devices, spanning from one address above the network address to one address below the broadcast address. For example, in the 192.168.1.0/24 network, the usable host range is 192.168.1.1 through 192.168.1.254.
Prefix Length: The number of consecutive 1-bits in a subnet mask, written after a slash (e.g., /24), that defines how many bits of an IP address represent the network portion versus the host portion. A /24 prefix means the first 24 bits identify the network, leaving 8 bits for host addresses.

Topics

Vlsm Subnetting Network Address Broadcast Address Interactive Calculation

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