VLSM Subnet Design for 190.100.0.0/16: Allocating 64×256, 128×128, 128×64 Addresses

We are given an IPv4 CIDR block $190.100.0.0/16$, which contains $2^{32-16}=65{,}536$ total addresses. CIDR uses the “slash notation” to indicate the network prefix length; the remaining bits determine the number of addresses in the block.2

We must use VLSM to carve the /16 space into subblocks that can satisfy each customer-group requirement, then determine how many addresses remain unallocated.

Key idea: a CIDR prefix $/p$ corresponds to a block size of $2^{32-p}$ addresses (total addresses, including network/broadcast in the raw count).

Footnotes

1. RIPE NCC, “Understanding IP Addressing and CIDR Charts — RIPE Network Coordination Centre” (https://www.ripe.net/about-us/press-centre/understanding-ip-addressing) - Explains IPv4 CIDR slash notation and how prefix length relates to bits. ↩

2. Wikipedia, “Classless Inter-Domain Routing (CIDR)” (https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing) - Describes CIDR syntax and prefix-length meaning. ↩

3. ITU Online IT Training, “Calculating CIDR Ranges for Efficient IP Address Management” (https://www.ituonline.com/blogs/calculating-cidr-ranges-for-efficient-ip-address-management) - Provides $32-$prefix host-bit relationship and example block sizing. ↩

Step 0: Convert each requirement to the smallest power-of-two block size (CIDR)

For a required number of addresses $A$, we choose the smallest CIDR block size $2^{k}$ such that $2^{k}\ge A$. The resulting prefix length is $p=32-k$.

We will also use subnet-mask sizing relationships as commonly shown in VLSM materials (e.g., /24→256 addresses, /25→128 addresses, /26→64 addresses).2

Address-to-prefix mapping

• $256$ addresses $\Rightarrow 256 = 2^{8} \Rightarrow p=32-8=24$ → $/24$
• $128$ addresses $\Rightarrow 128 = 2^{7} \Rightarrow p=32-7=25$ → $/25$
• $64$ addresses $\Rightarrow 64 = 2^{6} \Rightarrow p=32-6=26$ → $/26$

These are consistent with standard classless subnet sizing charts.

Footnotes

1. ComputerNetworkingNotes.com, “VLSM Subnetting Examples and Calculation Explained” (https://www.computernetworkingnotes.com/ccna-study-guide/vlsm-subnetting-examples-and-calculation-explained.html) - Discusses network/broadcast impacting usable hosts vs block size. ↩

2. Das Blinken Lichten / subnetting resources (https://www.dasblinkenlichten.com/subnetting-a-subnet-vlsm) - Shows subnet-mask sizing table including /24=256, /25=128, /26=64 relationships. ↩ ↩²

Calculations: total allocated addresses

Group (a): 64 customers, each needs 256 addresses

Each customer needs a $256$-address CIDR block: $/{24}$ (block size $256$).

Total allocated: $64 \\times 256 = 16{,}384$

Group (b): 128 customers, each needs 128 addresses

Each customer needs a $128$-address CIDR block: $/{25}$ (block size $128$).

Total allocated: $128 \\times 128 = 16{,}384$

Group (c): 128 customers, each needs 64 addresses

Each customer needs a $64$-address CIDR block: $/{26}$ (block size $64$).

Total allocated: $128 \\times 64 = 8{,}192$

Grand total allocated

$16{,}384 + 16{,}384 + 8{,}192 = 40{,}960$

Addresses remaining in 190.100.0.0/16

Total in /16: $65{,}536$ (since $2^{32-16}=65{,}536$)2

Remaining: $65{,}536 - 40{,}960 = 24{,}576$

Footnotes

1. Das Blinken Lichten / subnetting resources (https://www.dasblinkenlichten.com/subnetting-a-subnet-vlsm) - Shows subnet-mask sizing table including /24=256, /25=128, /26=64 relationships. ↩ ↩² ↩³

2. RIPE NCC, “Understanding IP Addressing and CIDR Charts — RIPE Network Coordination Centre” (https://www.ripe.net/about-us/press-centre/understanding-ip-addressing) - Explains IPv4 CIDR slash notation and how prefix length relates to bits. ↩

3. ITU Online IT Training, “Calculating CIDR Ranges for Efficient IP Address Management” (https://www.ituonline.com/blogs/calculating-cidr-ranges-for-efficient-ip-address-management) - Provides $32-$prefix host-bit relationship and example block sizing. ↩

Subblock design (network ranges)

We allocate from the start of the given block, i.e., beginning at $190.100.0.0$ and using increasing subnet addresses. This follows the common sequential VLSM planning method.

We only need the boundaries for remaining-space computation, but we can still list the actual subnet boundaries per group at a high level:

Group 1 (/24 blocks): 64 subnets of /24

• Number of /24 blocks: $64$
• Each /24 is $256$ addresses, and consecutive /24 blocks increase by $256$ in the last octet.

So Group 1 consumes: $64 \\times /{24} \\text{ blocks} = 64 \\times 256 = 16{,}384 \\text{ addresses}$

In terms of the /16’s second+third octets, this corresponds to consuming:

• $190.100.0.0/16$ space up through $190.100.63.0$ as /24 chunks (i.e., the first 64 blocks).

Group 2 (/25 blocks): 128 subnets of /25

Next available boundary after consuming 64×/24 aligns at $190.100.64.0$.

• 128 blocks of /25 consumes: $128 \\times 128 = 16{,}384$ So Group 2 consumes up to $190.100.127.255$.

Group 3 (/26 blocks): 128 subnets of /26

Next available boundary: $190.100.128.0$.

• 128 blocks of /26 consumes: $128 \\times 64 = 8{,}192$ So Group 3 consumes up to $190.100.159.255$.

Remaining address space

The overall /16 is $190.100.0.0$ through $190.100.255.255$. If the first $160$ of the $256$ possible “/24 chunks” worth of space are consumed (0–159), remaining is “/16 from 160 to 255”, which has:

• $(256-160)=96$ “/24 chunk equivalents”
• $96 \\times 256 = 24{,}576$ addresses

This matches the arithmetic remainder above.

Footnotes

1. Comparitech, “Variable Length Subnet Mask (VLSM) Tutorial - Fully Explained” (https://www.comparitech.com/net-admin/variable-length-subnet-mask-vlsm-tutorial) - Outlines VLSM procedure including allocating from largest needs and selecting appropriate subnets sequentially. ↩