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 21 hours ago by emotional_soup_88@programming.dev to c/linux@lemmy.ml
16 comments fedilink hide all child comments
 

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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[–] emotional_soup_88@programming.dev 10 points 21 hours ago (1 children)

Just when you think you know something about networking, it turns out you don't know sh*t. XD

Thank you for your exquisite explanation and for immediately realizing what I had been misunderstanding!

[–] RIotingPacifist@lemmy.world 1 points 20 hours ago* (last edited 15 hours ago) (1 children)

I'm not sure how useful this is, but it helped me understand.

The subnet mast is the count of the bits from the front that matter.

This is useful for switches and network equipment that only have to look at that much if the packet and route it quickly.

So a 10.0.0.0/8 address the switch only looks at the first 8 binary digits (also called an octet) and routes it based on that.

This also works for non multiple of 8 masks, as it's an operation done on a binary level (it's a mask using a binary AND so only bits that are true and.the mask can be matter)

10.192.0.0/10

IP: 00001010.11000000.00000000.00000000

Mask: 11111111.11 000000.00000000.00000000

So 10.192.0.1 and 10.192.255.255 look the same with their mask on

1: 00001010.11 000000.00000000.00000001

255: 00001010.11 111111.11111111.11111111

Mask: 11111111.11 000000.00000000.00000000

Result for both is: 00001010.11 000000.00000000.00000000

[–] scott@lem.free.as 2 points 15 hours ago (1 children)

Your masks are backward.

Ones on the left, zeroes on the right.

The mask is binary AND'd with the IP, leaving the leftmost octets alone.

[–] RIotingPacifist@lemmy.world 1 points 15 hours ago

That makes a lot more sense thanks