Backup SQL Server to different subnet drive

I know it is permission issue and not sure if it is right forum for it. If someone can guide please

SQL Server 2005 backup databases on server local drive having subnet 192.168.1.xx and server is not in domain

Lack of space on server local drive backup has to move on different server local drive which is on domain network. The subnet is different 10.15.18.55

I have create folder and permission to everyone on 10.15.18.xx

I can browse folder from 192.168.1.xx (SQL Server) to 10.15.18.xx and add/delete folders and files on shared drive

If I run database backup I cannot backup database from 192.168.1.xx subnet (SQL Server) to 10.15.18.xx subnet (Domain Server)

Error message

TITLE: Microsoft SQL Server Management Studio
------------------------------

Backup failed for Server 'itsdb'.  (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=9.00.5000.00&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Backup+Server&LinkId=20476

Muhammad Mehdi