You never execute the command. Also, you should be using using statements for proper memory disposal. Something like this:
using (SqlConnection sqlCon = new SqlConnection(connectionString){
sqlCon.Open();
using (SqlCommand cmd = new SqlCommand ("INSERT INTO test VALUES (@ip)", sqlConn){
cmd.Parameters.AddWithValue("@ip", textBox1.Text);
cmd.ExecuteNonQuery();
}
}