How to Define Default Client in SAP?

You can set the SAP to default client login via parameter "login/system_client" means every time when you are going to login in SAP you will find 100 as client as shown in below image

 This can be done using transaction code RZ10 . As an example, you can set the parameter to client 100 in transaction code RZ10. A restart is needed to make the changes activated.