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Is specifying meaningless bits valid syntax in CIDR notation? For instance, the netmask of 192.168.13.0/16 is 255.255.0.0. In this case, the last two octets (00001101 00000000) are meaningless because any value between 192.168.0.0 and 192.168.255.255 (inclusive) is in range. I'm curious what's considered valid in CIDR notation.

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    Hello and welcome. They're not "meaningless bits", they're "host bits" ! – jonathanjo Jul 16 '19 at 16:31
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Yes it's a valid syntax. Every host on a network has an IP address, and it's also important to know what subnet it's a part of. You need that information to configure the host. So 192.168.13.0/16 tells you what the host IP is and also what the subnet is.

If we follow your idea to the logical extreme, everything is in the range of 0.0.0.0/0, so we don't need to specify IP addressing at all.

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When you specify an address longer than the bits in the mask, you are generally referring to a host address. In your example, 192.168.13.0/16 would be a host on the 192.168.0.0/16 network. That gives you the host address and mask (similar to 192.168.13.0 255.255.0.0, except in shortened form), from which you can derive all type of network information. See this two-part answer for the details.

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