CCNA technique 010: Timed subnetting — practice and verify

Detailed practical guide

Prepare your workspace and inputs

Start with host 192.0.2.77/27, a documentation-only address. Before running the script, write the network, broadcast, first host, last host and usable host count. Set a timer if measuring recall speed; correctness is the first pass criterion.

Platform context: Paper and Python 3.10+ with ipaddress. No router changes or simulator license required.

Scenario to reproduce or investigate

You must configure a host and gateway in the same /27 without assigning the network or broadcast address. A familiar last octet is not enough; determine the actual address block.

1. Calculate the block on paper

   A /27 leaves five host bits. The last-octet block size is 32, so boundaries are 0, 32, 64, 96 and so on. Address 77 belongs to the block beginning at 64. Write the five required values before revealing the script result.

   Command, configuration or input

   Input: 192.0.2.77/27
   Mask: 255.255.255.224
   Last-octet block size: 32

   Expected result · ผลที่ควรเห็น

   Your prediction identifies 192.0.2.64/27, not 192.0.2.77/27 as the network address.

   If it fails · เมื่อไม่ผ่าน

   If you subtract 27 from 32 and use 5 as the increment, convert the five host bits to 2**5 addresses first.

2. Run the independent address check

   Save as subnet_check.py and run python3 subnet_check.py. The program checks both the original host and a changed host across the next block boundary. It also rejects a proposed gateway in the wrong subnet.

   Save and run this example

   from ipaddress import ip_interface, ip_address
   
   interface = ip_interface("192.0.2.77/27")
   network = interface.network
   hosts = list(network.hosts())
   assert str(network) == "192.0.2.64/27"
   assert str(network.broadcast_address) == "192.0.2.95"
   assert str(hosts[0]) == "192.0.2.65"
   assert str(hosts[-1]) == "192.0.2.94"
   assert len(hosts) == 30
   assert ip_address("192.0.2.65") in network
   assert ip_address("192.0.2.97") not in network
   changed = ip_interface("192.0.2.97/27").network
   assert str(changed) == "192.0.2.96/27"
   print("Network 192.0.2.64/27; broadcast 192.0.2.95")
   print("Usable 192.0.2.65-192.0.2.94; hosts 30")
   print("Changed host 192.0.2.97 belongs to 192.0.2.96/27")
   

   Expected result · ผลที่ควรเห็น

   The original usable range is .65 through .94, with 30 hosts. The changed .97 host belongs to the .96/27 network.

   If it fails · เมื่อไม่ผ่าน

   If Python raises a parsing error, check the slash notation and address octets. Do not generalize the subtract-two host rule to /31 or /32 networks; those have distinct semantics.

3. Translate the answer into a configuration check

   For a simulator lab, choose host .77/27 and a permitted gateway such as .65 in the same isolated topology. Verify the configured mask and gateway before testing a remote network. A local-subnet calculation does not prove that the gateway interface exists or that routing beyond it works. Repeat the paper calculation with .126/27 before looking at Python.

   Command, configuration or input

   Record: configured address | mask | gateway | expected local network | observed route

   Expected result · ผลที่ควรเห็น

   Your simulator or worksheet uses a host address and gateway from the same usable range and treats gateway reachability as a separate test.

   If it fails · เมื่อไม่ผ่าน

   If a correctly addressed host cannot reach a local gateway, check interface state, VLAN and link before changing the subnet calculation.

Recovery and cleanup

No network is modified by the script. If you use a simulator, save its original topology before edits and restore that file after the address exercise.

Apply to actual work and known limits

Use the same calculation for real assigned addresses, then verify actual VLAN, interface, gateway and route configuration. Documentation-only addresses in this exercise are not destinations to probe on the public internet.

The included self-contained Python fixture was executed successfully in this Linux environment. External products and production infrastructure were not exercised.

Network 192.0.2.64/27; broadcast 192.0.2.95
Usable 192.0.2.65-192.0.2.94; hosts 30
Changed host 192.0.2.97 belongs to 192.0.2.96/27