Networking – Why can’t IP addresses start with 0 or 255

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It was specified somewhere (presumably in an RFC) that Class A addresses range from 1-126 in the first octet, and that class E addresses range from 240-254, but that leaves out two potential networks on either side, namely a 0 and a 255 network.

I believe that addresses starting with these IPs are invalid. Does this mean that they wouldn't work on a private network, and is there a reason for them being invalid?

Best Answer

The IP address 0.0.0.0 is used as a wildcard to bind to all addresses. The IP address 255.255.255.255 is used to indicate a local broadcast.

Because this decision was part of the classful network design that predates CIDR, to reserve these specific IP addresses alone, special rules would have had to have been created for the /8's that contained them. Since some /8's were going to remain reserved anyway, it made sense to reserve these rather than creating additional rules so they could be used.

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