help

I have 3 systems with static ip address working in a LAN.
System 1 - Has a class A ip address 10.156.10.1
System 2 - Has a class A ip address 10.156.10.2
System 3 - Has a class A ip address 11.157.11.5

Can I add a ip address 11.157.11.0 to have the system on the networked with each other.Lets says that changing the Ip address is ruled out for system 3.

In order to enable communication between the two subnets, you will need to use a router. This could take the form of a standalone device or you could add a second NIC to System 3 and give it a static address in the 10.156.10.x range. Doing this may however introduce security risks depending on your setup.