Networking/Subnet question

Hi all
hopefully a quick question
I have two Airport Extremes
One is serving as my main router (latest Gen APE)
and it has my imac and a few other devices hardwired into it.

I also have an older APE working in bridge mode ("extend a wireless network)

Here is my question.
If i go into airport utility, the subnet mask for the newer APE is
255.255.254.0
on my older APE In bridge mode, its listed as 255.255.255.0
Does it matter which subnet my devices are set to?
for example. the imac that is hardwired into the new APE with the subnet of 255.255.254.0 has the 255.255.255.0 listed in the imac's system prefs/network/ethernet settings. Should that be switched to .254? does it make a difference?

thanks

yes, the 2nd/older APE is in "extend a wireless network" mode.