this post was submitted on 03 Mar 2026
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The behavior of /24 vs /32 addresses when using iptables (programming.dev)
submitted 22 hours ago by emotional_soup_88@programming.dev to c/linux@lemmy.ml
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I added a rule to accept connections from 192.168.1.135/24, since my router is configured to hand out /24 addresses. Then, iptables -L -v showed that connections from 192.168.1.0/24 are accepted. When I change the rule to accept connections from .135/32 - or from .135 without specifying the subnet -, it not only works as intended, but it also resolves the hostname correctly.

Why?

unsolicited "why do you still use iptables" advice not welcome :D

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[–] jrgd@lemmy.zip 3 points 20 hours ago* (last edited 20 hours ago) (1 children)

Iproute2 definitely does write things a bit compact. ip address show and shorthands state the routed local address space (192.168.1.x/24) and the actual /32 address (192.168.1.214) you are assigned as one unit. Additionally, it shows the broadcast address for the space. Ironically, ip route show may genuinely give you less confusing information, clearly splitting the actual route and showing your straight IPv4 address as src.

Typically in firewalling, you'd use /32 to target a singular IPv4 host. This is analogous to using /128 for IPv6 hosts. You can absolutely use /24, /16, /8, or any other mask really if you need to target a range of IP addresses for a rule to apply to. Technically, /32 is a range itself, just with a size of 1. There are CIDR calculators available to play around and see what different CIDR masks actually target.

[–] emotional_soup_88@programming.dev 2 points 20 hours ago

Sweet! Thanks for another clear explanation! Have good rest of your day! :)