My understanding is that means vs means to

For example in /etc/network/interfaces, /24 is used when configuring static IPs.

But in /etc/iptables/rules.v4, -d means any IP in the subnet... which equals to right?

Sorry for the newbie question... I am confused!

  • Thanks Michael, but that answer is for rocket scientist. Can someone please answer this like if I was 5? I’m sure others will be thankful. – sunknudsen Aug 7 at 21:20
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    That is the ELI5 explanation. – Michael Hampton Aug 7 at 21:22
  • What about a ELI1 then? 🤯 Trust me Michael, I spend thousands of hours teaching technology to people... that answer is still to meta. But thanks for pointing it out... I will jump into the rabbit hole. – sunknudsen Aug 7 at 21:33
  • @MichaelHampton Please see answers bellow and consider not closing this question. – sunknudsen Aug 7 at 21:53
  • Do you happen to know what a netmask is (eg If so, a /xx is an abbreviated way of writing a netmask – davidgo Aug 8 at 8:56

In simple terms, means that you set on your network interface but you are alone, the whole network is just your IP.

On the other hand, setting means that your IP is and the IP range is a known, familiar, reachable block of IPs; no gateway (that is, another network device) needed to reach it.

One real scenario of /32 subnet setting is in firewall rules, if you want just one IP to be allowed or blocked, not a range. In most cases typing just the IP in a firewall rule assumes /32. You wouldn't use it in a PC's network interface for everyday use.

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  • Thanks for your help! Does this mean that using /32 in /etc/network/interfaces would break connectivity to the rest of the network? Please see update question for context. – sunknudsen Aug 7 at 21:39
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    Yes, it would break connectivity. Regarding in iptables rules: That actually means the range ( .0 represents the network, .255 broacast - packet sent everywhere, they cannot be assigned to net devices). But in /etc/network/interfaces is not the same; it means setting IP, subnet mask on the net interface. – Krackout Aug 7 at 21:47
  • So this is confusing in the context of /etc/network/interfaces vs iptables right? The / notation has different meaning. – sunknudsen Aug 7 at 21:51
  • No, it's not different meaning; you'll have to dive a bit to understand it. Remember, .0 represents the whole network, it's not an ordinary IP. That's the difference. On the other hand, on other subnet masks, /23 for example, .0 can also be an ordinary IP, so be careful on assumptions! Not easy to explain further in a comment. – Krackout Aug 7 at 22:02

When your interfaces file has something like address, that specifies the address and the "netmask" (in the form of the /n network prefix length) all in one go.

Previously, the interfaces file would have had separate address and netmask entries in that same situation. It's simply a modernization to consistently use the /n notation.

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