0.0.0.0/0 CIDR - what is that, and why?

If you ever configured routing in cloud, or locally on the machine - you have probably seen it.

What is CIDR?
I will do more detailed post later, but this is the most common way of defining BUNCH of IPs in single entry.

How it works?
When working with IP - it is very nice to be able to operate on the bits rather than 4 numbers split by dots.

You have the following format <ip>/<mask>, e.g. 1.2.3.0/24.
What it all means? /24 part is how many leftmost bits you will take for that CIDR (in that case 24 bits / 8 -> first 3 octets of ip: 1.2.3|.X). This helps you to understand whether concrete IP is in that CIDR or not, for example for routing.

e.g. for 1.2.3.0/24
1.2.3.1 & 255.255.255.0 = 1.2.3.0 (TRUE)

1.2.4.1 & 255.255.255.0 = 1.2.4.0 (FALSE)

Now, what the hell is 0.0.0.0/0? why /0?
/0 - means we take ZERO leftmost bits, so no matter what address it is "ANDing" (&) it will give you 0. So it means essentially ANY ADDRESS

#kernel #network

Following up my own question I asked myself. Why 0.0.0.0?

Any address /0 (taking ZERO leftmost bits) will give ZERO. So we can as well define it as 1.1.1.1/0 or 1.2.3.4/0??

The thing is that it is enforced by linux kernel for routing table. Linux implement Longest Prefix Match (TRIE) for these CIDRs, and canonically host part should be zeroed: https://www.rfc-editor.org/rfc/rfc9164.html#section-4.3

It means our earlier example 1.2.3.0/24 will be rejected if defined like that 1.2.3.100/24 (however technically still it works)

To play with routes/CIDRs: https://tools.andreyka26.com/ip-checker

#kernel #network