CCNA/subnetting question

[adam2016]

Hi guys just wondering if anybody has any experience with the CCNA

I am following this youtube tutorial https://www.youtube.com/watch?v=bfFRiGygHns

basically we have to assign the addresses to the rest of the subnets,we have two subnets to assign addresses to,172.31.30.0/23

in the video the guy assigns routers G0/O interface 172.31.30.128 and a subnet mask of 255.255.255.128

and G1/0 interface to 172.31.30.254 subnet of 255.255.255.128

that looks wrong to me

but from my math in a /23 subnet you get 2 subnets of 510 hosts each,now maybe he subnetted the subnets? thats where he gets 255.255.255.128?

thanks

[helios]

That looks like an error to me. 255.255.255.128 is a 25-bit mask, not 23-.

Also, a /23 subnet gives you 512 addresses.