First, the essential ideas
- Bit and octet
- A bit is one binary digit, 0 or 1. An octet contains eight bits and can represent a decimal value from 0 to 255.
- IPv4 address
- A 32-bit network address, commonly written as four decimal octets, assigned to an interface; one device can have several interfaces and addresses.
- Interface and host
- An interface is a network connection on a device. A host is an endpoint that sends or receives data, such as a computer.
- Prefix and subnet mask
- The prefix length counts the leading network bits. A contiguous mask represents those bits as ones followed by host-bit zeros.
- Subnet
- An IP address block defined by a network address and prefix length. The host uses its mask to decide whether a destination belongs to its local subnet.
- Network and broadcast
- For an ordinary IPv4 LAN subnet, all-zero host bits identify the network and all-one host bits identify its directed broadcast, not ordinary host assignments.
- Usable host range
- The addresses between network and broadcast in the ordinary LAN examples here. Special /31 point-to-point and /32 cases use different rules.
- Default gateway
- A router's local address used for destinations without a more specific route. No gateway is configured in this isolated same-subnet lab.
- VPCS and console
- VPCS is GNS3's lightweight Virtual PC Simulator. Its console is a text window opened by right-clicking the named PC and choosing Console.
- ICMP and ping
- ICMP is an IP control-message protocol. Ordinary ping sends echo requests and reports replies or missing replies; it does not prove application health.
- Baseline and rollback
- A baseline records the intended settings and working observations. Rollback restores those exact settings after a controlled test.
- VLSM
- Variable Length Subnet Masking: using different prefix lengths for different child address blocks within an identified parent allocation.
A layered explanation is useful only when the basic words are clear. Start with the definitions below, then separate the paper calculation from the actual device test. An address plan predicts which settings belong together; the isolated experiment checks a particular configured path. Neither is evidence that an application or production security policy works.
See the idea
One /24, three subnet allocations
Allocate aligned, non-overlapping ranges and distinguish spare hosts from space reserved for future subnets.
Size the requirements
All blocks belong to 192.168.10.0/24. Choose /26, /27 and /28 for total usable requirements of 50, 20 and 10, including one planned gateway in each total.
What this model leaves out: This paper allocation omits traffic and physical devices. The separate GNS3 lab reuses PC1, Switch1 and PC2 for one subnet at a time; no routed departments or security isolation are demonstrated.
Full visual explanation and references
Within 192.168.10.0/24, Sales uses .0/26, Engineering .64/27 and Management .96/28. The .112/28 and .128/25 blocks remain reserved; this is an allocation, not a packet path.
This paper allocation omits traffic and physical devices. The separate GNS3 lab reuses PC1, Switch1 and PC2 for one subnet at a time; no routed departments or security isolation are demonstrated.
- Size the requirements
All blocks belong to 192.168.10.0/24. Choose /26, /27 and /28 for total usable requirements of 50, 20 and 10, including one planned gateway in each total.
www.cisco.comwww.rfc-editor.org - Check every full range
Allocated intervals are .0-.63, .64-.95 and .96-.111. The starts are aligned, the ranges do not overlap, and network and broadcast addresses are not ordinary hosts.
www.cisco.comwww.rfc-editor.org - Reserve future blocks
Keep .112/28 and .128/25 unallocated. Sales cannot expand in place to .0/25 without overlapping Engineering and Management; growth still needs a reviewed plan.
www.cisco.comwww.rfc-editor.org
One idea at a time
Start with something familiar.
No router command knowledge is assumed. Read the definitions first and write the address plan on paper. A console is the text window opened by right-clicking the named GNS3 PC and choosing Console; do not type commands into the switch or your computer's operating-system terminal.
Why learn this?
A plan can look tidy while assigning a network address to a router, overlapping another subnet or leaving no room where expansion is needed. Checking capacity, alignment and full ranges before configuration prevents those avoidable mistakes.
An everyday comparison
As a memory aid, imagine reserving different-sized numbered seating sections for groups of different sizes. Label which seats are assigned and which sections are held for future groups. This is not a literal model of packet forwarding.
What happens in a network
Different child prefixes create different address capacities inside a parent allocation. A reservation must be documented so another operator does not allocate the same range again.
Where the comparison stopsReal address blocks must start on binary prefix boundaries; you cannot choose arbitrary group sizes or slide them freely like seats. The analogy cannot prove routing, connectivity or security isolation.
Follow one worked example
- Write the address and its prefix together.
The same decimal address can belong to different-sized blocks when its prefix changes. Copy both values before doing arithmetic; do not infer the mask from an old class label.
Why: An address without the configured mask is incomplete evidence for a subnet calculation.
- Calculate boundaries before assigning a host.
Find the network, last address and ordinary usable host range. Check that the example host lies inside the intended block, not on a reserved LAN boundary.
Why: Planning first makes off-by-one errors visible before you type a value into a device.
- Compare the plan with actual observations.
Configure both PCs for Sales and test the remote peer. Reconfigure both PCs for Engineering, then Management, testing each phase separately. Finally restore both Sales addresses and repeat the original ping. Record actual results without inventing a successful transcript.
Why: Three sequential local tests connect each calculated subnet to observed settings and connectivity. They do not demonstrate routed departments or production applications.
Your first small task
On paper, list three total usable-address requirements: Sales 50, Engineering 20 and Management 10. These totals already include a future gateway address. Choose an ordinary LAN prefix for each before looking at the allocation example.
Show a hint
Choose the smallest power-of-two total block whose ordinary host count, after subtracting network and broadcast, meets the requirement.
Check the expected result
A /26 supplies 62 usable addresses, a /27 supplies 30, and a /28 supplies 14; all three meet their respective stated totals.
Pause and explain it in your own words
Can Sales expand from .0/26 to .0/25 without reviewing the other assignments?
Show a hint
Compare the full .0-through-.127 proposed range with the Engineering and Management allocations.
Show the explanation
No. The expanded /25 would cover existing .64/27 and .96/28 assignments. Free space elsewhere in the parent does not remove that overlap; expansion needs a revised plan and potentially renumbering.
Short answer
What should you understand today?
VLSM uses different prefix lengths inside a parent block. Count required addresses, choose aligned blocks large enough for each need, check full ranges for overlap and document reserved growth space. Free addresses elsewhere do not guarantee that an existing subnet can expand in place.
Before you begin
Learning targets
- Review the earlier endpoint and Ethernet lessons; all required addressing terms are defined here before the teaching body.
- Use GNS3 with its included VPCS and Ethernet switch nodes. No Cisco router image or IOS commands are required.
- Work in a fresh isolated project with no physical, Cloud, NAT or production-network connection.
- Have a plain-text notes application and a calculator or paper available; the calculation steps are explained before the lab.
- Count address requirements including gateway reservations and planned growth, not just user devices.
- Allocate aligned unequal subnet blocks and identify network, host and broadcast boundaries.
- Check parent containment, complete-range overlap and free-space reservations.
- Distinguish sequential local subnet tests from simultaneous routed networks or security isolation.
Words you will meet
Open a word when you need its meaning. You do not need to memorize the whole list before reading.
Bit and octet
A bit is one binary digit, 0 or 1. An octet contains eight bits and can represent a decimal value from 0 to 255.
IPv4 address
A 32-bit network address, commonly written as four decimal octets, assigned to an interface; one device can have several interfaces and addresses.
Interface and host
An interface is a network connection on a device. A host is an endpoint that sends or receives data, such as a computer.
Prefix and subnet mask
The prefix length counts the leading network bits. A contiguous mask represents those bits as ones followed by host-bit zeros.
Subnet
An IP address block defined by a network address and prefix length. The host uses its mask to decide whether a destination belongs to its local subnet.
Network and broadcast
For an ordinary IPv4 LAN subnet, all-zero host bits identify the network and all-one host bits identify its directed broadcast, not ordinary host assignments.
Usable host range
The addresses between network and broadcast in the ordinary LAN examples here. Special /31 point-to-point and /32 cases use different rules.
Default gateway
A router's local address used for destinations without a more specific route. No gateway is configured in this isolated same-subnet lab.
VPCS and console
VPCS is GNS3's lightweight Virtual PC Simulator. Its console is a text window opened by right-clicking the named PC and choosing Console.
ICMP and ping
ICMP is an IP control-message protocol. Ordinary ping sends echo requests and reports replies or missing replies; it does not prove application health.
Baseline and rollback
A baseline records the intended settings and working observations. Rollback restores those exact settings after a controlled test.
VLSM
Variable Length Subnet Masking: using different prefix lengths for different child address blocks within an identified parent allocation.
Alignment and overlap
Alignment means a block begins on a valid boundary for its prefix. Overlap means two allocated ranges contain at least one common address.
Reservation
A documented address or block intentionally kept unavailable for ordinary assignment, such as a future gateway or a future subnet.
Count the actual address requirement
Start with a written parent allocation and the number of usable addresses each group needs. Include user endpoints, any planned gateway and other infrastructure; do not silently count a gateway twice or forget it. In this worked example the parent is 192.168.10.0/24. Sales needs 50 usable addresses in total, Engineering 20 and Management 10; those totals already include one future gateway reservation per group. Choose ordinary LAN blocks whose usable capacity is large enough. A /26 has 64 total addresses and 62 ordinary usable hosts, a /27 has 32 and 30, and a /28 has 16 and 14. There are therefore 12, 10 and 4 spare usable addresses within these allocations. These are planning numbers, not claims that every possible real deployment has the same reserved-address rules.
- State the parent prefix explicitly.
- Count infrastructure and growth requirements consistently.
- Do not confuse total block size with usable host capacity.
Allocate aligned blocks from largest to smallest
In an initially empty parent, placing larger requirements first is a useful planning heuristic because they need larger aligned spaces. It is not a universal optimization guarantee for an already fragmented plan. Allocate Sales to 192.168.10.0/26, which spans .0 through .63. Next allocate Engineering to 192.168.10.64/27, spanning .64 through .95. Allocate Management to 192.168.10.96/28, spanning .96 through .111. The starts are aligned to increments of 64, 32 and 16 respectively. A subnet cannot begin at any convenient next host number: for example, .97 is a valid Management host but not the /28 network boundary. The first and last addresses of each block remain reserved as network and broadcast in these ordinary LAN examples.
- Use aligned network starts, not arbitrary host addresses.
- The Management network .96/28 is not a host assignment.
- Check the whole block through its broadcast, not only the first usable host.
Check overlap and parent containment
Two different written network names can still describe overlapping ranges. Compare the complete first and last address of each allocation. Our three blocks are adjacent and disjoint: .0-.63, .64-.95 and .96-.111. All lie inside the parent .0-.255. Now reject a proposed 192.168.10.64/26 addition: it spans .64-.127 and overlaps both Engineering and Management. Similarly, 192.168.10.30/27 belongs to .0-.31 and overlaps Sales, not Engineering. These are paper checks; do not deliberately install an invalid address on an unfamiliar router to see whether it rejects it. Router validation varies by context, and a device accepting a command does not prove that the overall organization's allocation is correct.
- Check full intervals, including reserved boundary addresses.
- Two different prefixes can still overlap.
- A device command's acceptance is not an organization-wide allocation audit.
Reserve growth where it can actually be used
After the three allocations, .112 through .255 remains unallocated: 144 total addresses. Represent that remainder as two aligned blocks, 192.168.10.112/28 and 192.168.10.128/25. They are separate reservations, not one automatically assignable subnet. Label their intended use and owner in the plan. Spare hosts inside Sales allow growth up to its present usable capacity, but increasing Sales from /26 to /25 would consume .0-.127 and overlap the other allocated blocks. Expansion may require renumbering, relocation or an earlier design that reserves adjacent aligned space. There is no promise of automatic growth merely because VLSM is used. A useful review asks which group is expected to expand, by how much and whether its specific adjacent space is still free.
- Unallocated address count and immediately expandable capacity are different.
- Document reservation ownership and intended purpose.
- Recheck alignment and overlap before enlarging any existing prefix.
Test the plan without inventing three routed networks
This beginner lab uses two VPCS nodes and one built-in Ethernet switch. Reuse that exact isolated path for three sequential phases: first Sales, then Engineering, then Management. Readdress both endpoints before testing each phase and record the result; do not mix old and new endpoint settings. The future gateway addresses .1, .65 and .97 are reservations on paper only, not active routers. The lab observes local reachability inside each selected block. It does not demonstrate simultaneous departments, communication between the blocks, VLAN separation, routing protocols, firewall behavior or growth under load. Those need a different topology and later lessons. If an application or security result was not tested, state that limitation instead of implying it from a successful ping.
- Only one subnet is active on the two-PC path at a time.
- Inspect both endpoints after every readdressing phase.
- A local ICMP reply does not prove routed connectivity or policy isolation.
Real-world walkthrough
An address-plan review before a Day 10 lab change
A hypothetical training room needs an address plan that a second operator can check. A learner has written only a computer address and says it must be correct because it looks familiar. The reviewer asks for the prefix, boundary calculation, intended peer and a rollback record. This is a planning scenario, not a claim that the isolated lab implements an entire office network.
- Write the complete address and prefix for each proposed endpoint and identify the parent address block the exercise is allowed to use.
- Calculate the network, ordinary host range and broadcast before assigning anything; compare the results with the intended block and check for duplicate host addresses.
- Record exact lab cable endpoints and initial settings. Keep a copy of the baseline outside the device so an incorrect setting can be restored accurately.
- Configure only the two virtual endpoints in the isolated project, inspect both settings and test the remote peer. Keep actual output separate from expected results.
- Test the Sales, Engineering and Management address pairs in separate sequential phases, changing both PCs for each phase. Restore both Sales addresses and repeat the original remote ping. Record unexplained results without claiming inter-subnet routing or security isolation.
Guided GNS3 lab
Test three VLSM host ranges one at a time
Validate the paper allocation and inspect local peer communication in three sequential subnet phases; do not claim simultaneous routing, VLAN separation or application performance.
Addressing plan
| Device | Interface | Address | Purpose |
|---|---|---|---|
| PC1 (Sales phase) | Ethernet0 | 192.168.10.2/26 | First phase; future gateway .1 is not implemented. |
| PC2 (Sales phase) | Ethernet0 | 192.168.10.3/26 | Remote peer in Sales .0/26. |
| PC1 (Engineering) | Ethernet0 | 192.168.10.66/27 | Replaces PC1's Sales address during phase two. |
| PC2 (Engineering) | Ethernet0 | 192.168.10.67/27 | Replaces PC2's Sales address during phase two. |
| PC1 (Management) | Ethernet0 | 192.168.10.98/28 | Replaces PC1's Engineering address during phase three. |
| PC2 (Management) | Ethernet0 | 192.168.10.99/28 | Replaces PC2's Engineering address during phase three. |
| Switch1 | Ethernet0 and Ethernet1 | No IP address required | Same two isolated cables throughout all phases. |
Set up the lab
- Create a fresh GNS3 project; use a configured local server or GNS3 VM appropriate to your installation.
- Drag two built-in VPCS nodes from End devices and a built-in Ethernet switch from Switches. Rename them PC1, PC2 and Switch1.
- Connect the exact interfaces in the topology. Start both PCs and leave the switch ports on their default access VLAN 1.
- Do not add a Cloud, NAT or physical network adapter. Perform these tests only in the isolated lab, never on production or shared traffic.
- Open Notepad on Windows, TextEdit in plain-text mode on macOS, or a plain-text editor on Linux. Save your baseline and observations as Day10-address-plan.txt.
Validate the allocation
On paper, record Sales .0/26, Engineering .64/27 and Management .96/28 within 192.168.10.0/24. Write every network, usable interval and broadcast. Check alignment, containment and disjoint intervals; reserve .112/28 and .128/25 without assigning them.
- What you should see
- The allocated ranges end at .63, .95 and .111 respectively; the two growth reservations together contain 144 unallocated addresses.
- Why this step matters
- A checked plan establishes the boundaries before any endpoint is configured. It also prevents a network address such as .96/28 being assigned as an ordinary host.
Configure PC1 for Sales
Right-click PC1 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.2/26
show ipWhat each command means
ip 192.168.10.2/26- Assign PC1 address 192.168.10.2/26, replacing the previous phase's setting without adding a gateway.
show ip- Display PC1's actual address and mask and compare them with the Sales row of the plan.
- What you should see
- PC1 shows 192.168.10.2 and mask 255.255.255.192. It is inside the Sales usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Configure PC2 for Sales
Right-click PC2 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.3/26
show ipWhat each command means
ip 192.168.10.3/26- Assign PC2 address 192.168.10.3/26, replacing the previous phase's setting without adding a gateway.
show ip- Display PC2's actual address and mask and compare them with the Sales row of the plan.
- What you should see
- PC2 shows 192.168.10.3 and mask 255.255.255.192. It is inside the Sales usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Verify Sales peer
Right-click PC1 and choose Console before typing. Check that both PCs have the current phase's addresses, then test the other PC at 192.168.10.3, not the sender's own address. Repeat if an initial request times out; investigate persistent loss and record actual output.
ping 192.168.10.3 -c 3What each command means
ping 192.168.10.3 -c 3- Send three ICMP echo requests to the remote peer 192.168.10.3; -c 3 bounds the count and does not test an application or another subnet.
- What you should see
- Expect replies in the correctly configured Sales phase. This result applies only to the named local pair, not to simultaneous departments or a routed path.
- Why this step matters
- Comparing the planned host interval with actual remote replies provides bounded operational evidence without inventing a router, gateway or complete office network.
Configure PC1 for Engineering
Right-click PC1 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.66/27
show ipWhat each command means
ip 192.168.10.66/27- Assign PC1 address 192.168.10.66/27, replacing the previous phase's setting without adding a gateway.
show ip- Display PC1's actual address and mask and compare them with the Engineering row of the plan.
- What you should see
- PC1 shows 192.168.10.66 and mask 255.255.255.224. It is inside the Engineering usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Configure PC2 for Engineering
Right-click PC2 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.67/27
show ipWhat each command means
ip 192.168.10.67/27- Assign PC2 address 192.168.10.67/27, replacing the previous phase's setting without adding a gateway.
show ip- Display PC2's actual address and mask and compare them with the Engineering row of the plan.
- What you should see
- PC2 shows 192.168.10.67 and mask 255.255.255.224. It is inside the Engineering usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Verify Engineering peer
Right-click PC1 and choose Console before typing. Check that both PCs have the current phase's addresses, then test the other PC at 192.168.10.67, not the sender's own address. Repeat if an initial request times out; investigate persistent loss and record actual output.
ping 192.168.10.67 -c 3What each command means
ping 192.168.10.67 -c 3- Send three ICMP echo requests to the remote peer 192.168.10.67; -c 3 bounds the count and does not test an application or another subnet.
- What you should see
- Expect replies in the correctly configured Engineering phase. This result applies only to the named local pair, not to simultaneous departments or a routed path.
- Why this step matters
- Comparing the planned host interval with actual remote replies provides bounded operational evidence without inventing a router, gateway or complete office network.
Configure PC1 for Management
Right-click PC1 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.98/28
show ipWhat each command means
ip 192.168.10.98/28- Assign PC1 address 192.168.10.98/28, replacing the previous phase's setting without adding a gateway.
show ip- Display PC1's actual address and mask and compare them with the Management row of the plan.
- What you should see
- PC1 shows 192.168.10.98 and mask 255.255.255.240. It is inside the Management usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Configure PC2 for Management
Right-click PC2 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.99/28
show ipWhat each command means
ip 192.168.10.99/28- Assign PC2 address 192.168.10.99/28, replacing the previous phase's setting without adding a gateway.
show ip- Display PC2's actual address and mask and compare them with the Management row of the plan.
- What you should see
- PC2 shows 192.168.10.99 and mask 255.255.255.240. It is inside the Management usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Verify Management peer
Right-click PC1 and choose Console before typing. Check that both PCs have the current phase's addresses, then test the other PC at 192.168.10.99, not the sender's own address. Repeat if an initial request times out; investigate persistent loss and record actual output.
ping 192.168.10.99 -c 3What each command means
ping 192.168.10.99 -c 3- Send three ICMP echo requests to the remote peer 192.168.10.99; -c 3 bounds the count and does not test an application or another subnet.
- What you should see
- Expect replies in the correctly configured Management phase. This result applies only to the named local pair, not to simultaneous departments or a routed path.
- Why this step matters
- Comparing the planned host interval with actual remote replies provides bounded operational evidence without inventing a router, gateway or complete office network.
Reject an overlap on paper
Without changing any device, evaluate proposed block 192.168.10.64/26. Calculate its full .64-through-.127 interval and compare it with the existing plan. Then examine whether Sales could expand to .0/25 without moving other allocations.
- What you should see
- The proposed .64/26 overlaps Engineering and Management. Sales .0/25 also overlaps both, so neither change is approved by this plan.
- Why this step matters
- An overlap is an address-range fact, not a test that requires deliberately entering invalid commands into a device. Free space elsewhere does not prove safe in-place expansion.
Configure PC1 for Sales baseline
Right-click PC1 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.2/26
show ipWhat each command means
ip 192.168.10.2/26- Assign PC1 address 192.168.10.2/26, replacing the previous phase's setting without adding a gateway.
show ip- Display PC1's actual address and mask and compare them with the Sales baseline row of the plan.
- What you should see
- PC1 shows 192.168.10.2 and mask 255.255.255.192. It is inside the Sales baseline usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Configure PC2 for Sales baseline
Right-click PC2 and choose Console before typing. Confirm the named endpoint, then replace its previous address with the current phase's value. Enter each command separately and record the actual address and mask. Do not run the phase test until both PCs have been configured.
ip 192.168.10.3/26
show ipWhat each command means
ip 192.168.10.3/26- Assign PC2 address 192.168.10.3/26, replacing the previous phase's setting without adding a gateway.
show ip- Display PC2's actual address and mask and compare them with the Sales baseline row of the plan.
- What you should see
- PC2 shows 192.168.10.3 and mask 255.255.255.192. It is inside the Sales baseline usable range, not on its network or broadcast.
- Why this step matters
- Inspecting each endpoint independently prevents a mixed-phase configuration from being mistaken for an invalid VLSM calculation.
Verify restored Sales baseline peer
Right-click PC1 and choose Console before typing. Check that both PCs have the current phase's addresses, then test the other PC at 192.168.10.3, not the sender's own address. Repeat if an initial request times out; investigate persistent loss and record actual output.
ping 192.168.10.3 -c 3What each command means
ping 192.168.10.3 -c 3- Send three ICMP echo requests to the remote peer 192.168.10.3; -c 3 bounds the count and does not test an application or another subnet.
- What you should see
- Expect replies in the correctly configured restored Sales baseline phase. This result applies only to the named local pair, not to simultaneous departments or a routed path.
- Why this step matters
- Comparing the planned host interval with actual remote replies provides bounded operational evidence without inventing a router, gateway or complete office network.
Verify
- Check all three capacities against the stated total requirements, including the already-counted gateway reservation.
- Verify block alignment and the complete intervals .0-.63, .64-.95 and .96-.111; no pair may intersect.
- Record both actual endpoint settings and the named remote result for each sequential phase before readdressing again.
- Confirm .1, .65 and .97 remain paper gateway reservations only; no gateway or router was configured.
- Confirm the unallocated remainder is .112/28 plus .128/25, not one arbitrary 144-address subnet.
- Verify both PCs return to the Sales baseline and the final remote test receives replies before stopping the project.
Troubleshoot
- If a console cannot open, start the named VPCS node and check GNS3's console application. Switch1 is a built-in Ethernet switch, not an IOS device.
- If a phase fails, inspect both show ip outputs. Both PCs must use that phase's prefix; do not test while one still has an earlier phase's address.
- Inspect the two cable endpoints and default access VLAN 1. No router or Internet connection should be added as a workaround.
- An initial timeout can accompany address discovery; persistent loss requires investigation. On real networks, filtering, hosts, rate limits and links can also affect ping.
- If the plan overlaps, stop before implementation and revise the complete allocation record. Do not rely solely on whether a device accepts a command.
- Perform all troubleshooting and restoration only in this isolated project, not on shared or production traffic.
Built-in VPCS and Ethernet switch nodes require no Cisco image. GNS3 does not provide Cisco images. Use Cisco images only when the applicable license permits the intended use; do not share or redistribute image files. Cisco Modeling Labs is the official Cisco alternative.
- Restore PC1 .2/26 and PC2 .3/26 as shown in the final steps and record the final remote test result.
- Keep the allocation table, reservations and phase observations in Day10-address-plan.txt. Save the topology and stop the project.
- Do not save or change any production settings. When reopening, inspect or re-enter the documented baseline because VPCS settings may not persist without saving.
Practice set
Answer before you reveal
01Which ordinary LAN prefix satisfies 50 total usable-address requirements?
A /27 has only 30 usable hosts and is too small. The total already includes the future gateway, so do not add it twice or describe all 62 addresses as user endpoints.
02Can 192.168.10.96/28 be assigned as the Management router's ordinary interface address?
Management spans .96 through .111, with ordinary hosts .97 through .110. The practical lab does not instantiate a router or gateway, but its paper reservation must still be valid.
03What is the unallocated remainder after the three example allocations?
The allocations consume 64 + 32 + 16 = 112 addresses of the /24's 256. The remainder is represented by aligned blocks, not an arbitrary single subnet of 144 addresses.
04Why does proposed 192.168.10.64/26 overlap the plan?
Overlap is determined from complete intervals, not just different prefix labels. Reject the plan on paper before attempting any device change.
05Does free space at .128/25 let Sales .0/26 grow in place automatically?
Growth must fit adjacent aligned space or require a revised plan and possible renumbering. Free addresses elsewhere do not create an unoccupied larger block at the existing start.
06What do the three sequential GNS3 phase tests demonstrate?
They do not implement simultaneous departments, inter-subnet routing, VLAN isolation or a firewall. The same two PCs and switch are readdressed between phases.
Knowledge check
Quiz: prove the reasoning
Keep these
Technical takeaways
- VLSM allocates unequal aligned blocks inside an explicit parent.
- Count all usable-address requirements, including infrastructure and planned growth.
- Network and broadcast boundaries are not ordinary host assignments in these LAN examples.
- Check full intervals for overlap and containment before configuration.
- Reserve future blocks explicitly; free addresses do not guarantee in-place expansion.
- Sequential local tests do not establish routed connectivity or security isolation.