Subnetting…..but in reverse

About this video

A CCNA lesson on reverse subnetting: starting from an already-configured host and working backwards to find its network address, broadcast address and usable range in order to troubleshoot it. The setup is a host, Beatrice, who cannot ping anything, presented in a lab built with Boson NetSim, the series sponsor Boson Software. Because the mask is already given, the usual first steps are skipped. He converts the mask 255.255.240.0 to binary, counts 20 network bits for a /20, and finds the increment is 16, occurring unusually in the third octet rather than the fourth. He then builds ranges in steps of 16 and places Beatrice’s address in the second network, noting that an address ending in 255 can still be a valid host in a large network because the broadcast is simply the last address of that range. Checking the other host and the router shows the fault: they sit in the first network while Beatrice sits in another, and her default gateway 172.17.0.1 is not inside her own network, so she can reach nothing. Viewers are challenged to run ipconfig or ifconfig, reverse-subnet their own device and phone, and solve the episode’s opening exam-style question.

Key takeaways

• Reverse subnetting starts from a configured IP, mask and gateway and derives the network address, broadcast address and range
• For mask 255.255.240.0 he counts a /20 and an increment of 16, applied in the third octet
• An address ending in 255 or 0 is not automatically invalid; validity depends on the size and boundaries of its network
• The troubleshooting payoff: Beatrice could not ping because her gateway 172.17.0.1 lay outside her own network, unlike the other host and router
• He urges continued practice on viewers’ own devices and frames the problem as typical of CCNA-style exam questions supported by Boson practice exams and NetSim labs

Episode details: published July 21, 2023 · 8:26 long. Summary written by Self Growth Videos from the episode itself. Business results and any income figures mentioned are NetworkChuck’s own claims, shared for education — they are not typical or guaranteed.

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