In configuring a IPSec site to site vpn with SRX 240 we need to set the st0/1/2 Adapters to manual address

For this I choose 172.27.0.0 Subnet 30 which only gives 2 IP’s per subnet (between SRX1 and SRX2)

If you try and assign an IP in the Broadcast Address or Subnet Address wou will get

Cannot assign broadcast address as ip address

or

Cannot assign address 0 on subnet

Use a subnet caculator for checking these address’ and only use the values in between the Min and Max Host

http://wintelguy.com/subnetcalc.pl