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Because you get issued a range and you actually do whatever you want within that range. Like IP addresses. What you do within that range is up to up, you don't

Because you get issued a range and you actually do whatever you want within that range. Like IP addresses. What you do within that range is up to up, you don't actually have to ask the authority for every single number within the range. As a simplification: If you get the number from 100 to 200 to assign to your books but the last digit is a check digit, you didn't get 100 numbers to choose from. You got to choose from: 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and the third number is the check digit, which is computed from the other numbers. Just with ISBNs (and other numbering schemes) the numbers are larger of course. Going back to the IP address analogy: if you are issued 11.0.0.0/8, you can use anything from 11.0.0.1 to 11.255.255.255 but you don't have to start with 11.0.0.1 and you don't have to treat it as one big /8 either. You can subdivide it into multiple /28s for example. The only difference here is that with ISBNs (or anything else that has a check digit) is that the last digit is not freely usable. It's just a check digit. It's not really part of the range so to speak. It's like you subdividing your /8 into lots of /28s but ignoring the fact that the network address on the /28s is actually the network address and having one of the computers in your network use it as their actual IP address. That's obviously gonna wreak some havoc because some of your network thinks it's a network address for one of the /28s and some others think it's an individual computer.

bambax 4y ago
To elaborate on your example, if the last digit cast out nines from the previous two digits, valid numbers are 101 (1+0=1), 112, 123, 134, 145, 156, 167, 178, 189, 191 (1+9=10, 1+0=1), 202. So for example 102 is NOT a valid number — it doesn't exist! And if it is used, and someone somewhere tries to validate it, it will throw an error. Hence the article says: > Shel very quickly removed all ‘checksum software checks’ (which would have made sure it was a legal ISBN) I find it incredibly fascinating (and a little bit depressing) that publishers would twist the system in that way, but maybe it has to be expected if something is sold for a large amount of money. In France ISBN are free; there is a central agency (Electre) that issues them but they don't charge for it (at least not to small publishers). But EAN/UPC codes are issued by a different agency, GS1, that does charge a recurring fee, and it is expensive and "infinite" (you're supposed to keep paying for as long as you're using the codes). But even them don't charge for the range size; the fee is based on the turnover of the company buying the codes.

bambax 4y ago

To elaborate on your example, if the last digit cast out nines from the previous two digits, valid numbers are 101 (1+0=1), 112, 123, 134, 145, 156, 167, 178, 189, 191 (1+9=10, 1+0=1), 202. So for example 102 is NOT a valid number — it doesn't exist! And if it is used, and someone somewhere tries to validate it, it will throw an error. Hence the article says: > Shel very quickly removed all ‘checksum software checks’ (which would have made sure it was a legal ISBN) I find it incredibly fascinating (and a little bit depressing) that publishers would twist the system in that way, but maybe it has to be expected if something is sold for a large amount of money. In France ISBN are free; there is a central agency (Electre) that issues them but they don't charge for it (at least not to small publishers). But EAN/UPC codes are issued by a different agency, GS1, that does charge a recurring fee, and it is expensive and "infinite" (you're supposed to keep paying for as long as you're using the codes). But even them don't charge for the range size; the fee is based on the turnover of the company buying the codes.