Connecting to Oracle takes more than 15 seconds

Could anybody suggest me what may be the problem here. It takes between 15 - 25 seconds to connect to oracle database?

thaks for any help on this issue!

well if you use hostname you have to define your hosts file otheriwse the DNS takes longer

We use TCP IP address on all our files

how´s host defined in tnsnames.ora

Here is entry from clients tnsnames.ora file

VDR5.world =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 10.2.27.4)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = VDR5)
)
)

How much time tnsping takes?