is your database local(ie,,in the same machine?).
if so,
as per your posting
[i]conn rajesh/rajesh@kritidb[/i]
your are using a connect string KRITIDB
1. FOR A LOCAL DATABASE YOU NEED TO USE A CONNECT STRING.
  You can connect as rajesh/rajesh.
2. IF you want to use a connect string, then,
   edit the $ORACLE_HOME/network/tnsnames.ora file
   and change value of your host to LOCALHOST.
   something like this 
kritidb =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = < substitute_your_sid > )
    )
  )

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The issue is becuase, DSL uses floating IP.
so when you are connected to DSL the value of HOST is not the same as when you are NOT CONNECTED.
change it to LOCALHOST.
and port=1521 is the default.
if you use other, specify that.