Re-subnet your lab and watch the mask decide everything

applied · 50 min · Objective 1.7

Task

Split your lab's 192.168.10.0/24 into four /26 subnets, move a host into a different one, and observe the failures a wrong mask produces. The exam asks about incorrect subnet masks as a symptom; this lab gives you the symptom first-hand so the question answers itself.

Steps

  1. Work out the four /26 subnets of 192.168.10.0/24 on paper, with their network, first host, last host and broadcast addresses.
  2. Reconfigure Host C into the second /26 while leaving it on the same physical segment: sudo ip addr flush dev eth0 then sudo ip addr add 192.168.10.70/26 dev eth0. Host A stays at 192.168.10.10/24.
  3. From Host A, ping Host C. Then from Host C, ping Host A. Note that one direction may work and the other may not, and work out why from the two masks. Host A believes C is local; C believes A is on a different network and sends to a gateway that does not exist in its subnet.
  4. Now create the classic partial failure. Put Host C back on 192.168.10.11 but with the wrong mask: /26 instead of /24. From C, ping 192.168.10.10 (works — same /26) and then 192.168.10.100 (fails — C thinks it is remote). That "some hosts reach, some do not" pattern is the exam signature of a wrong mask.
  5. Restore both hosts to /24 and confirm full connectivity.

Verify

ip -br addr show eth0
ip route show
ping -c 2 -W 1 192.168.10.10; echo "near exit $?"
ping -c 2 -W 1 192.168.10.100; echo "far exit $?"

With Host C on a /26, the near ping must succeed and the far one must fail — two different results from one interface, which is the whole diagnostic. After restoring /24, both behave consistently.

Notes

The reason this fault is so confusing in the field is that it is not symmetrical and not total. Half of what the user tries works. They report "the network is flaky", which sends you looking for a physical problem, and the actual cause is one number in one config file.

The general rule this teaches: when connectivity is selective rather than absent, suspect the mask. Absent connectivity is a link, a route or a firewall; selective connectivity points at the host's own idea of what is local.