Backport IPv4-lowest-address-as-ordinary-unicast-address?

dtaht · April 10, 2022, 12:37am

I simply don't do enough openwrt development these days to be able to test this -2 line patch which is in linux mainline now. Help?

github.com

schoen/unicast-extensions/blob/master/merged-patches/linux/0001-Treat-IPv4-lowest-address-as-ordinary-unicast-address.patch

From c92f9caeececaaad9461da47592d01a9a8111f2c Mon Sep 17 00:00:00 2001
From: Seth David Schoen <schoen@loyalty.org>
Date: Fri, 25 Sep 2020 16:29:30 -0700
Subject: [PATCH net-next 1/2] ip: Treat IPv4 lowest address as ordinary unicast address.

Treat the lowest address in a subnet (the address within the subnet
which contains all 0 bits) as an ordinary unicast address instead
of as a potential second broadcast address.  For example, in subnet
192.168.17.24/29, which contains 8 addresses, make address 192.168.17.24
usable as a normal unicast address (while continuing to support
192.168.17.31 as a broadcast address).

Since EVERY network number or subnet formerly had its host number 0
reserved, this patch adds 1 more usable host address to every network
and subnet (i.e., 2^(32-n)-1 instead of 2^(32-n)-2 addresses available
for assignment on each IPv4 /n subnet).  For small subnets, this is a
significant gain; instead of 6 usable host addresses, a /29 would now
contain 7, a 16% increase.

The reserving of host number 0 for broadcast came about in RFC 1122 from

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But gaining that extra IPv4 address for heavily subnetted networks seems valuable so if there is anyone out there that can try this?

How to test: backport the patch to the current kernel.

assign thyself a .0 address and see if it works.
assign thyself a CIDR x.y.z.32/27 and see if it works.

There may be issues with other applications but so far we haven't found any. The gui might reject .0 as an address, as another example.