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What is the wildcard mask for a /23?

Answer
0.0.1.255 wildcard for /23 (255.255.254.0)

A /23 has the subnet mask 255.255.254.0. Subtract it from 255.255.255.255 and you get the wildcard 0.0.1.255: the last 9 bits are "don't care", so it matches 512 consecutive addresses.

wildcard · adjust the values

The calculation

255.255.255.255 − 255.255.254.0 = 0.0.1.255

A /23 block is typically two contiguous /24s, for example a VLAN that outgrew 254 hosts. With the network 10.0.0.0, the wildcard 0.0.1.255 matches 10.0.0.0 through 10.0.1.255.

Using it in Cisco IOS

! standard ACL
access-list 10 permit 10.0.0.0 0.0.1.255

! named extended ACL
ip access-list extended FROM-SITE
 permit ip 10.0.0.0 0.0.1.255 any

! OSPF
router ospf 1
 network 10.0.0.0 0.0.1.255 area 0

Junos, Arista EOS, MikroTik and Linux use the prefix form instead: 10.0.0.0/23.

Nearby prefixes

PrefixSubnet maskWildcardAddresses
/21255.255.248.00.0.7.2552,048
/22255.255.252.00.0.3.2551,024
/23255.255.254.00.0.1.255512
/24255.255.255.00.0.0.255256
/25255.255.255.1280.0.0.127128

More questions like this

/16 → 0.0.255.255/17 → 0.0.127.255/18 → 0.0.63.255/19 → 0.0.31.255/20 → 0.0.15.255/21 → 0.0.7.255/22 → 0.0.3.255/24 → 0.0.0.255/25 → 0.0.0.127/26 → 0.0.0.63/27 → 0.0.0.31/28 → 0.0.0.15/29 → 0.0.0.7/30 → 0.0.0.3

Frequently asked questions

How many addresses does 0.0.1.255 match?

512. The wildcard has 9 one-bits, and 29 = 512.

Is 0.0.1.255 the same as 255.255.254.0?

No. 255.255.254.0 is the subnet mask; 0.0.1.255 is its bitwise inverse. Putting the subnet mask where IOS expects a wildcard matches the wrong addresses.