Is 0.0.0.0 a valid IP address?

Question

Is 0.0.0.0 a valid IP address? I want my program to be able to store it as an indication that no address is in use, but this won\'t work if it\'s actually valid.

Answer 1

As other answers have covered, 0.0.0.0 is a legal and valid for some purposes IP address.

If all values in a range are legal values, then any flag items you want to define must come from somewhere else. Otherwise you will overload a legal value with the meaning of a flag, and when using the overloaded value it won't be 100% possible to determine whether the legal value or the flag meaning was intended. This will lead to bugs which must be solved with a re-factor which can be an expensive endeavour.

Overloading legal values happens all the time, IP address and MAC addresses (yes, 00:00:00:00:00:00 is legal and allocated) are some of the most common victims.

Coincidentally, I am working on a system (not ethernet/IP based) now where the length of a frame is capped at about 40 bytes. Since a byte can represent lengths of 0 - 255 bytes, and the max length is 40 bytes, I can use some of the unused aka non-legal values (252 to 255) to represent flag items. These in-band flags are okay because there is no overloading.