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What is meant by fixed length sub-net masking? Is it mean Class A -----255.0.0.0 Class B -----255.255.0.0 Class C -----255.255.255.0

As i understand its mean is keep the length of the network prefix length constant. Is it right?

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    You could benefit more from the time you spent asking that. Just search on Google there are so many articles on that. You can start here: en.wikipedia.org/wiki/Classful_network – Alex Jul 11 '14 at 11:50
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Yes, but most importantly, classful addressing (i.e. fixed length subnet masks) is obsolete and has been for at least 20 years. I don't know why networking classes still teach it. There are many, many resources on this and other stack exchanges (as well as the Internet) that explain how to use variable length masks.

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    We learn about classful addressing, for example, to understand why BGP auto-summary function works that way. Yeah it's old too, but not going away. If you didn't know about classful addressing you would think, what the hell is going on here? – Alex Jul 11 '14 at 11:54
  • I know, but you could make the same argument for lots of barely-used Cisco commands. – Ron Trunk Jul 11 '14 at 12:04

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