Indicia

/30 subnet (255.255.255.252)

A /30 network has 4 addresses, of which 2 can be given to hosts after the network and broadcast addresses. Two usable hosts. The traditional choice for point-to-point links before /31 became common.

CIDR notation
/30
Subnet mask
255.255.255.252
Wildcard mask
0.0.0.3
Binary mask
11111111.11111111.11111111.11111100
Total addresses
4
Usable hosts
2
Example network
192.168.1.0/30
Example usable range
192.168.1.1 – 192.168.1.2
/30 networks in a /24
64

Calculate a /30 for your own address

192.168.1.0/24 split into /30 networks

NetworkUsable rangeBroadcast
192.168.1.0/30192.168.1.1 – 192.168.1.2192.168.1.3
192.168.1.4/30192.168.1.5 – 192.168.1.6192.168.1.7
192.168.1.8/30192.168.1.9 – 192.168.1.10192.168.1.11
192.168.1.12/30192.168.1.13 – 192.168.1.14192.168.1.15
192.168.1.16/30192.168.1.17 – 192.168.1.18192.168.1.19
192.168.1.20/30192.168.1.21 – 192.168.1.22192.168.1.23
192.168.1.24/30192.168.1.25 – 192.168.1.26192.168.1.27
192.168.1.28/30192.168.1.29 – 192.168.1.30192.168.1.31
192.168.1.32/30192.168.1.33 – 192.168.1.34192.168.1.35
192.168.1.36/30192.168.1.37 – 192.168.1.38192.168.1.39
192.168.1.40/30192.168.1.41 – 192.168.1.42192.168.1.43
192.168.1.44/30192.168.1.45 – 192.168.1.46192.168.1.47
192.168.1.48/30192.168.1.49 – 192.168.1.50192.168.1.51
192.168.1.52/30192.168.1.53 – 192.168.1.54192.168.1.55
192.168.1.56/30192.168.1.57 – 192.168.1.58192.168.1.59
192.168.1.60/30192.168.1.61 – 192.168.1.62192.168.1.63
192.168.1.64/30192.168.1.65 – 192.168.1.66192.168.1.67
192.168.1.68/30192.168.1.69 – 192.168.1.70192.168.1.71
192.168.1.72/30192.168.1.73 – 192.168.1.74192.168.1.75
192.168.1.76/30192.168.1.77 – 192.168.1.78192.168.1.79
192.168.1.80/30192.168.1.81 – 192.168.1.82192.168.1.83
192.168.1.84/30192.168.1.85 – 192.168.1.86192.168.1.87
192.168.1.88/30192.168.1.89 – 192.168.1.90192.168.1.91
192.168.1.92/30192.168.1.93 – 192.168.1.94192.168.1.95
192.168.1.96/30192.168.1.97 – 192.168.1.98192.168.1.99
192.168.1.100/30192.168.1.101 – 192.168.1.102192.168.1.103
192.168.1.104/30192.168.1.105 – 192.168.1.106192.168.1.107
192.168.1.108/30192.168.1.109 – 192.168.1.110192.168.1.111
192.168.1.112/30192.168.1.113 – 192.168.1.114192.168.1.115
192.168.1.116/30192.168.1.117 – 192.168.1.118192.168.1.119
192.168.1.120/30192.168.1.121 – 192.168.1.122192.168.1.123
192.168.1.124/30192.168.1.125 – 192.168.1.126192.168.1.127
192.168.1.128/30192.168.1.129 – 192.168.1.130192.168.1.131
192.168.1.132/30192.168.1.133 – 192.168.1.134192.168.1.135
192.168.1.136/30192.168.1.137 – 192.168.1.138192.168.1.139
192.168.1.140/30192.168.1.141 – 192.168.1.142192.168.1.143
192.168.1.144/30192.168.1.145 – 192.168.1.146192.168.1.147
192.168.1.148/30192.168.1.149 – 192.168.1.150192.168.1.151
192.168.1.152/30192.168.1.153 – 192.168.1.154192.168.1.155
192.168.1.156/30192.168.1.157 – 192.168.1.158192.168.1.159
192.168.1.160/30192.168.1.161 – 192.168.1.162192.168.1.163
192.168.1.164/30192.168.1.165 – 192.168.1.166192.168.1.167
192.168.1.168/30192.168.1.169 – 192.168.1.170192.168.1.171
192.168.1.172/30192.168.1.173 – 192.168.1.174192.168.1.175
192.168.1.176/30192.168.1.177 – 192.168.1.178192.168.1.179
192.168.1.180/30192.168.1.181 – 192.168.1.182192.168.1.183
192.168.1.184/30192.168.1.185 – 192.168.1.186192.168.1.187
192.168.1.188/30192.168.1.189 – 192.168.1.190192.168.1.191
192.168.1.192/30192.168.1.193 – 192.168.1.194192.168.1.195
192.168.1.196/30192.168.1.197 – 192.168.1.198192.168.1.199
192.168.1.200/30192.168.1.201 – 192.168.1.202192.168.1.203
192.168.1.204/30192.168.1.205 – 192.168.1.206192.168.1.207
192.168.1.208/30192.168.1.209 – 192.168.1.210192.168.1.211
192.168.1.212/30192.168.1.213 – 192.168.1.214192.168.1.215
192.168.1.216/30192.168.1.217 – 192.168.1.218192.168.1.219
192.168.1.220/30192.168.1.221 – 192.168.1.222192.168.1.223
192.168.1.224/30192.168.1.225 – 192.168.1.226192.168.1.227
192.168.1.228/30192.168.1.229 – 192.168.1.230192.168.1.231
192.168.1.232/30192.168.1.233 – 192.168.1.234192.168.1.235
192.168.1.236/30192.168.1.237 – 192.168.1.238192.168.1.239
192.168.1.240/30192.168.1.241 – 192.168.1.242192.168.1.243
192.168.1.244/30192.168.1.245 – 192.168.1.246192.168.1.247
192.168.1.248/30192.168.1.249 – 192.168.1.250192.168.1.251
192.168.1.252/30192.168.1.253 – 192.168.1.254192.168.1.255

In firewall rules and investigations

A /30 rule matches every address from 192.168.1.0 to 192.168.1.3. In Cisco ACLs the same range is written with the wildcard mask 0.0.0.3. When an alert names an IP, checking whether it falls inside a known /30 (your own network, a cloud provider’s published range or a blocked range) is often the quickest first step.