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VLSM

Time: 45 min

Needs: 1 OL9 Router, 4 hosts, 4 Ethernet switches

GNS3 project: w05-a3-vlsm

Info

Times listed are approximate and provided as timing guidelines only. Take as much time as you need to complete the activity.

The brief

You have been allocated one block: 172.16.40.0/25. You may not request more.

Segment Hosts required now Growth expected
OFFICE 50 to 60
LAB 20 to 25
SERVERS 6 to 12
MGMT 2 to 4

For transit to the upstream router, you have two addresses available: 203.0.113.0/31

Constraints:

  1. No overlap. Every subnet disjoint from every other.
  2. Each subnet must fit its growth figure plus the router's own address, and be the smallest prefix that does. A segment planned for 6 hosts needs 7 usable addresses — the router lives in the subnet too.
  3. Contiguous from the bottom of the block.
  4. State the unused remainder blocks (may be 0, 1 or more) as the smallest set of CIDR blocks that covers it exactly
  5. Use the first usable address in each subnet for the router's address.

Part 1 — The plan (do this before touching GNS3)

Allocate largest first.

Segment Hosts needed + router Prefix / Net ID / Brd ID First usable / Last usable Spare
OFFICE 60 61
LAB 25 26
SERVERS 6 7
MGMT 2 3
Transit 2 2
Unused remainder — —

Check before you build:

  • [ ] Every prefix is the smallest that fits its growth number plus one for the router
  • [ ] No two ranges overlap — check each boundary against the next network address
  • [ ] Total allocated + remainder = 128 addresses exactly
  • [ ] Each usable count ≥ the "+ router" column, not the "hosts needed" column
  • [ ] The remainder is expressed as properly aligned CIDR blocks, not as a range

Part 2 - Peer review (optional)

Swap plans with another student. Build their network from their document only — no conversation, no clarifying questions. Then record:
Could you build it without asking anything?
Anything ambiguous, and how you resolved it
Anything wrong — overlap, wrong size, wrong boundary
One thing their document did better than yours

Part 3 — Build it

One OL9 Router. Each segment — OFFICE, LAB, SERVERS and MGMT — gets its own Ethernet switch, named SW-OFFICE, SW-LAB, SW-SERVERS and SW-MGMT, with one host on it.

The transit subnet has not cable in this lab. Configure it on a spare interface

On R1: 

nmcli con add type ethernet ifname eth0 con-name office \
      ipv4.method manual ipv4.addresses 172.16.40.1/26
nmcli con up office

Do the remaining router interfaces for the remaining segments.

Then, enable IP Forwarding: sysctl -w net.ipv4.ip_forward=1

Configure the hosts (all hosts are VPCS nodes), making sure to set the default gateway.

For example, H1 on SW-OFFICE:

ip 172.16.40.11/26 172.16.40.1
save

Part 4 - Prove it

  • On one of the host, run ip route get against the other three hosts and satisfy yourself that each each host has a different route
  • Ping all four hosts and ensure all pings succeed.