# IPv4 subnetting and VLSM — optional worked example
Purpose: practice the subnetting portions of Networking Infrastructures NI-NB-07 and Network Design ND-NIC-07. These are locally assigned study IDs, not official FBLA identifiers. This is an original note, not an official guide or a complete treatment of either objective.

## Edition and scope
The Networking Infrastructures and Network Design scope follows the National 2026–27 CONNECT guidelines. This worked example covers subnetting portions of the study objectives, not either entire event. Use the event courses for broader preparation and confirm division-specific requirements with an adviser.

## IPv4 foundations
An IPv4 address is 32 bits. A subnet mask marks the network prefix with 1 bits and the host portion with 0 bits. CIDR /n means n leading network bits; the dotted mask is a readable form of those bits. Example: /26 = 255.255.255.192 because the final octet mask is 11000000.

Classful networking divided historical address ranges into fixed Class A/B/C network sizes. Modern routing uses classless CIDR prefixes: the prefix length, not an address's first octet class, defines the network boundary. Do not infer a subnet mask from an address's old class.

For an ordinary subnet, network address has all host bits zero; directed broadcast has all host bits one. Usable host range lies between them. Conventional usable count is 2^(32-prefix)-2, except special cases such as /31 point-to-point links (RFC 3021) and /32 host routes.

## Worked example: /26
Given 192.0.2.77/26, mask is 255.255.255.192. Block size in final octet = 256 - 192 = 64. Blocks start 0, 64, 128, 192; 77 belongs to block 64. Network 192.0.2.64; broadcast 192.0.2.127; usable hosts 192.0.2.65–192.0.2.126 (62 conventional hosts). Check: bitwise IP address AND mask gives the network address; setting all host bits to one gives the broadcast address.

## Worked VLSM plan
Allocate from 192.0.2.0/24, largest host requirement first. Need 100, 50, and 20 hosts. A /25 has 126 conventional hosts; assign 192.0.2.0/25 (usable .1–.126, broadcast .127). A /26 has 62; assign 192.0.2.128/26 (usable .129–.190, broadcast .191). A /27 has 30; assign 192.0.2.192/27 (usable .193–.222, broadcast .223). Remainder is 192.0.2.224/27. No ranges overlap; each start aligns to its block size. VLSM uses different prefix lengths within the parent block.

## Special cases and traps
/31 is valid for a two-endpoint point-to-point link under RFC 3021: both addresses are endpoints, with no separate network/broadcast address convention. Do not apply the ordinary minus-two rule there. /32 denotes one exact IPv4 address, commonly a host route; it is not an ordinary multi-host LAN. Confirm platform and context before applying special cases.
Common traps: subtracting two for every prefix; choosing blocks in small-to-large order; confusing mask value with block size; forgetting boundary alignment; including network/broadcast as ordinary host assignments; calling CIDR “classful”; allowing VLSM overlaps; and using public-looking real addresses in examples. 192.0.2.0/24 is TEST-NET-1 documentation space (RFC 5737), not a deployable public allocation.

## Retrieval practice — answer before opening answers
1. For 198.51.100.139/27, find mask, network, broadcast, usable range, and conventional host count.
2. Why does /26 use a block size of 64 in the final octet?
3. Allocate requirements 60, 25, and 10 hosts from 203.0.113.0/24 using VLSM; show prefix, range, and broadcast.
4. Explain why a /31 point-to-point link is not handled by the normal minus-two rule.
5. What does /32 represent, and why is it not a normal LAN subnet?

## Application questions
A small office has a lab allocation of 192.0.2.0/24. Segments need 90, 40, 12, and 2 assigned addresses, including any gateway/router interfaces. Propose a largest-first VLSM allocation, state assumptions about the two-address link, and prove there is no overlap or out-of-parent allocation.
An engineer says 198.51.100.139 historically falls in Class C and therefore must use /24. Correct the reasoning and explain what information actually determines the subnet.

## Answer key
1. /27 mask 255.255.255.224, block 32. Network 198.51.100.128; broadcast .159; usable .129–.158; 30 hosts.
2. 256 minus the final-octet mask value 192 equals 64 addresses per subnet.
3. 60: 203.0.113.0/26 (hosts .1–.62, broadcast .63); 25: .64/27 (hosts .65–.94, broadcast .95); 10: .96/28 (hosts .97–.110, broadcast .111). Remaining .112–.255 is unallocated.
4. RFC 3021 assigns both /31 addresses to the two link endpoints; no network or directed-broadcast address is reserved for that link convention.
5. /32 has zero host bits and identifies one address, often used for a route to one host.
Office: 90 needs /25 at .0–.127; 40 needs /26 at .128–.191; 12 needs /28 at .192–.207; two endpoint addresses can use .208/31 only if a point-to-point link and supporting equipment are intended. Otherwise allocate /30 (.208–.211), leaving .212–.255. All are aligned, disjoint, and within /24.
Classful reasoning is obsolete for modern subnet selection. The configured/advertised CIDR prefix determines the boundary; 10.4.8.9 alone does not.

## Packet Tracer lab
1. Represent three LANs sized for 100, 50, and 20 assigned addresses from the worked 192.0.2.0/24 block. Use one PC and switch per LAN; you do not need to place 170 devices. Add a router with three usable Ethernet interfaces, or an equivalent routed-VLAN design. Keep this documentation-only lab disconnected from public networks.
2. Use .1/25, .129/26, and .193/27 as the router's LAN gateways. Use .2/25, .130/26, and .194/27 on the respective PCs, each with its matching gateway. The required address counts include gateway interfaces.
3. Verify each PC can ping its local gateway, then ping PCs across LANs. In the router CLI inspect interfaces and routes; use Simulation mode to observe ARP and ICMP.
4. Deliberately configure one host with a wrong mask, then wrong gateway. Record the distinct symptom, restore settings, and retest. Verify no host uses a network or broadcast address and no subnet overlaps.
5. Save the .pkt locally. This learning note does not ask you to publish personal progress or lab files.

## Companion Anki prompts
The following prompts support subnetting practice for study references NI-NB-07 and ND-NIC-07. If using the companion FBLA package, locate the exact prompt in the named event subdeck through Anki Browse. Without the package, use these as additional recall questions. They address parts of the two objectives, not their full scope.

- Networking Infrastructures: "What does an IP prefix length distinguish within an address?" — prefix versus host bits.
- Networking Infrastructures: "How many conventional usable host addresses are in 192.0.2.0/26?" — ordinary host-count arithmetic.
- Networking Infrastructures: "What is the network address of 192.0.2.77/26?" — block alignment and network address.
- Networking Infrastructures: "The dotted-decimal subnet mask equivalent to /27 is ____." — prefix-to-mask conversion.
- Networking Infrastructures: "On a supporting two-endpoint IPv4 point-to-point link, how many usable endpoint addresses does a /31 provide?" — scoped /31 exception.
- Networking Infrastructures: "How many exact IPv4 addresses does 198.51.100.7/32 identify?" — single-address /32 prefix.
- Network Design: "Why put largest subnets first when planning a VLSM allocation?" — aligned, non-overlapping allocation reasoning.

After recall, solve retrieval questions with new values and complete the lab. Record mistakes privately, then revisit the relevant prompt. Matching a card to a subskill does not certify full guide coverage, technical performance in a lab, or personal mastery.

## Sources and verification
Primary standards: RFC 4632, Classless Inter-domain Routing (CIDR), https://www.rfc-editor.org/rfc/rfc4632; RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links, https://www.rfc-editor.org/rfc/rfc3021; RFC 5737, IPv4 Address Blocks Reserved for Documentation, https://www.rfc-editor.org/rfc/rfc5737. Cisco, Subnetting a Class C Network (legacy class terminology; use its arithmetic examples, not classful assumptions): https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html
The 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 examples are documentation-only TEST-NET ranges. This flowing worked example differs from the event courses' fixed-length colored sheets. In the companion Anki package, search for tag:Networking_Infrastructures or tag:Network_Design tag:Test to locate subnet and prefix questions. Tags help navigation; they do not measure mastery or establish complete objective coverage. The linked references explain the technical background and should be consulted for their full scope and qualifications.