8 thoughts on “CompTIA Network+ N10-004: 1.4 – Subnet Masks – Part 2”

  1. Same here, but it’s still alot easier than reading it in a book. I’d like to thank you James for the work you’re putting into this. I’ve been into computers and networks, linux, unix, bsd, etc. since i was about 8 years old, i’m 27 now and i’m finally getting around to getting certified.. Hopefully I can break into my dream IT career with your help.

    Thanks again for this, god bless you!

  2. Thank you, sir!
    I learned this today at school, but i wasnt able pay much attention and watching this video a few times have helped me


  3. leaves you with 31 and after factoring in broadcast and subnet address would give you 29 hosts (chart says 30). Am I miscalculating something?

  4. Great stuff! Thanks Prof!

    One question. Another set of material says that when you subnet, you effectively get two less subnets than the math says you do. Your chart says a /26 subnet mask gives you four networks with 62 possible hosts, which makes sense since 2^2 =4. The other material says that even though 2^2=4, only two of the networks can actually be used since, if I’m reading it correctly, a network ID cannot be all zeros or all ones, of course eliminating two options.

    I suspect the question is not “who is right” as much as it is “what am I missing here?”!

    1. Early on in TCP/IP, the industry thought it would be a bad thing to have both a subnet and a network with the same address. RFC 950 (August 1985) says that a subnet of all ones or all zeros can’t be used. This is where the problem started.

      As TCP/IP rolled out across LAN environments this was found to be a waste, with two perfectly useful address ranges going unused. In December of 1995, RFC 1878 called the practice “obsolete,” and the industry switched to using all available networks when subnetting. It’s not that the idea of removing all-ones and all-zeros is wrong, it’s just fifteen year-old thinking. The only reason you would ever avoid an all-ones or all-zeros subnet is if you had some 15 year-old equipment that didn’t know any better!

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