Here are more detail:

-- OS on clien and on server
Window NT on both client and server

-- type of network protocol
TCP/IP

-- what application u are using
sqlplus

-- sqlnet.ora on server and client mashines
-- Client sqlnet.ora file as below:
# SQLNET.ORA Network Configuration File: d:\Oracle\Ora81\network\admin\sqlnet.ora
# Generated by Oracle configuration tools.

NAMES.DEFAULT_DOMAIN = xxxx.xxxxxx.com

SQLNET.AUTHENTICATION_SERVICES= (NTS)

NAMES.DIRECTORY_PATH= (TNSNAMES)

-- server sqlnet.ora file as below:
AUTOMATIC_IPC = OFF
TRACE_LEVEL_CLIENT = OFF
names.directory_path = (TNSNAMES)
names.default_domain = world
name.default_zone = world


-- server listener.ora as follow
################
# Filename......: listener.ora
# Node..........: local.world
# Date..........: 24-MAY-95 13:23:20
################
LISTENER =
(ADDRESS_LIST =
(ADDRESS=
(COMMUNITY= TCP.world)
(PROTOCOL= TCP)
(Host= xx.xx.xx.xx)
(Port= 1521)
)
(ADDRESS=
(COMMUNITY= TCP.world)
(PROTOCOL= TCP)
(Host= xx.xx.xx.xx)
(Port= 1526)
)
)
STARTUP_WAIT_TIME_LISTENER = 0
CONNECT_TIMEOUT_LISTENER = 10
TRACE_LEVEL_LISTENER = ADMIN
SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(SID_NAME = ORCL)
)
(SID_DESC =
(SID_NAME = xxx)
)
(SID_DESC =
(SID_NAME = xxx)
)
(SID_DESC =
(SID_NAME = xxx)
)
)
PASSWORDS_LISTENER = (oracle)


thanks, cathy