Although material is freely available in the network, it is not necessarily freely usable in all possible ways. Material is very often protected by copyright.
Check the pages to see if there are terms how the page material may be used. If there are no terms you must consider what legislation could be applied to the material.
If the Finnish Copyright Act can be applied you can act as explained in How to use copyright protected material within teaching.
If the Finnish legislation cannot be applied you must either find out what is stated in the legislation that is applied to the material or ask the creator permission to use the material. (See How to acquire rights and co-operation projects). You can also ask the organisation granting licenses if the material may be used according to some license. (See Licenses and organisations looking after the rights).
One who takes material as part of his/her own creation is responsible for asking permission to use the material. Creator can, however, direct his claims of infringement to anyone who is using their copyright protected material without proper authorisation.
If you restrict access to the material by putting it behind password it does not make the use more permissible.
Linking pages using a direct link which leads you to another pages can be regarded as permissible. Other links are problematic. Good custom is to ask the web page owner if you can make links and you should ask permission especially when you are thinking of using other links than just links directing to another page.