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I'm taking an AWS Lab to better familiarize myself with creating a VPC.

The instructions describe the example network we are to create in the lab like so:

enter image description here

My background is in software, not networking, so the second paragraph sent me on a hunt to refresh my networking knowledge so that I can understand more clearly what I'm creating and how it works.

I stumbled upon this excellent video on TCP/IP addressing and it states that 10.0.0.0 is a Class A network which has a default subnet mask of 255.0.0.0, but the lab I'm taking says:

The VPS itself is a Class B network in the 10.0.0.0 space...[with a subnet] of 255.255.0.0.

So what gives? How is the VPC a Class B network but it is in a Class A space (10.0.0.0)?

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  • keep reading/studying. Any time you see anything about "classful" networking, you're still decades in history. Learn CIDR and ignore classful; also ask why you're bother to be shown classful. This may also help, CIDR also, all homework/study questions are off-topic here... – Craig Constantine Dec 31 '15 at 19:04
  • It's free Amazon labs for learning AWS systems ... not actual homework / study questions. – doremi Jan 1 '16 at 17:56
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First, classful routing has been dead for 20 years, replaced by CIDR and VLSM in 1995, and is really only useful as a historical reference.

The class of an address is determined by the first bits of the address. Some people mistakenly have decided that any /8 is a Class A, any /16 is a Class B, and any /24 is a Class C. It's true those are the default network sizes for those classes, but that doesn't mean that 10.0.0.0/16 is a Class B network.

  • Class A addresses start with 0 as the first bit.
  • Class B addresses start with 10 as the first two bits.
  • Class C addresses start with 110 as the first three bits.
  • Class D addresses start with 1110 as the first four bits.
  • Class E addresses start with 1111 as the first four bits.
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Just to add one footnote to @RonMaupin's answer: although classful routing is long dead, many people informally use the terms "Class A, B and C" as shorthand for subnet masks. So when someone calls a network a Class A network, they simply mean it has a mask of /8 or 255.0.0.0. Similarly, when they call something a Class B or Class C network, they merely mean it has a mask of /16 or /24, respectively. That is the sense it is used in the quote you posted.

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  • The server guys at work drive me crazy with that. They often insist on a "Class C" network. I try to explains what they are asking for, but they won't listen and tell me that they really do know what a Class C network is and absolutely must have a Class C network, So I get one for them, but then they aren't happy because it isn't in the Class A address space. Then I get a lecture from the server guy who asked for it explaining to me that any network with a mask of 255.255.255.0 is a Class C network, and I go through my explanation again, but I get told I'm wrong. – Ron Maupin Dec 31 '15 at 17:47
  • Yeah, I feel the same way, but I got tired of pushing that rock up the hill ;-) – Ron Trunk Dec 31 '15 at 18:04

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