11g alert.log appeared a lot of Fatal NI connect error 12170.

 we notice in 11g alert.log

a lot of " Fatal NI connect error 12170" 

such as

"Fatal NI connect error 12170.
 Tracing not turned on.
  Tns error struct:
    ns main err code: 12535
Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=XXX.XXX.X.YY)(PORT=22772))  "

my interest is that  does we should say it's a client or network problem ,  not oracle server 's listener process problem ,

i notice a lot of client of connect to db , just this ip such as XXX.XXX.X.YY  report this problem, so is that my opinion right ?