taap.digitalnetwork engineering toolkit

What is the wildcard mask for a /24?

Answer
0.0.0.255 wildcard for /24 (255.255.255.0)

A /24 has the subnet mask 255.255.255.0. Subtract it from 255.255.255.255 and you get the wildcard 0.0.0.255: the last 8 bits are "don't care", so it matches 256 consecutive addresses.

wildcard · adjust the values

The calculation

255.255.255.255 − 255.255.255.0 = 0.0.0.255

A /24 block is typically a single standard VLAN or LAN segment. With the network 10.0.0.0, the wildcard 0.0.0.255 matches 10.0.0.0 through 10.0.0.255.

Using it in Cisco IOS

! standard ACL
access-list 10 permit 10.0.0.0 0.0.0.255

! named extended ACL
ip access-list extended FROM-SITE
 permit ip 10.0.0.0 0.0.0.255 any

! OSPF
router ospf 1
 network 10.0.0.0 0.0.0.255 area 0

Junos, Arista EOS, MikroTik and Linux use the prefix form instead: 10.0.0.0/24.

Nearby prefixes

PrefixSubnet maskWildcardAddresses
/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
/26255.255.255.1920.0.0.6364

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 → 0.0.3.255/23 → 0.0.1.255/25 → 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 0.0.0.255 match?

256. The wildcard has 8 one-bits, and 28 = 256.

Is 0.0.0.255 the same as 255.255.255.0?

No. 255.255.255.0 is the subnet mask; 0.0.0.255 is its bitwise inverse. Putting the subnet mask where IOS expects a wildcard matches the wrong addresses.