/23 subnet: mask 255.255.254.0, 510 usable hosts

How /23 works

With 23 network bits, the last bit of the third octet belongs to the host part. The mask is 255.255.254.0, the wildcard 0.0.1.255, and a /23 always starts on an even third octet.

2^9 = 512 addresses, 510 usable. For 10.0.0.0/23 the range is 10.0.0.1 to 10.0.1.254 and the broadcast 10.0.1.255. The boundary between the two halves, 10.0.0.255 and 10.0.1.0, consists of two normal host addresses.

Growing a /24 into a /23

When a 254-host LAN fills up, widening it to a /23 doubles capacity without renumbering existing hosts, provided the adjacent /24 is free and the original /24 has an even third octet. 192.168.4.0/24 can grow to 192.168.4.0/23; 192.168.5.0/24 cannot, because its /23 would start at .4.

Every host, the gateway and the DHCP server must learn the new mask. A host left on 255.255.255.0 will think the other half is off-link and send that traffic to the router, which mostly works until it does not.

Cloud and splitting notes

On AWS or Azure a /23 subnet leaves 507 assignable IPs. Split it into two /24s or four /25s; pair two aligned /23s and you have a /22.

Configuring the wider mask

Interfaces take 10.0.0.1/23 or the dotted 255.255.254.0. Update the DHCP scope so the pool can cross from the first /24 into the second; leaving it unchanged gives you a /23 on paper with the same 254-address pool in practice.

Firewall objects and monitoring that referenced the old /24 also need widening, or traffic from the new half will be dropped or go unwatched. Search the rule base for the old network address before the change, not after the first ticket arrives.

Going the other way is harder. Shrinking a /23 back to a /24 means first moving every host out of the half you give up, so check the DHCP lease table and ARP caches for stragglers before changing the mask.