There are 2 choices:
1. A centralised hardware or software solution in your network. Like you can have a centralised firewall or centralised antivirus. The company that maintains your network can advise.
2. Install parental control software on each PC. That's not so very expensive. Since your employees maybe are smarter than the kids it's meant for you, should make them very clear that messing with it themselves on their PC will be a reason to fire them and have them formally agree with that. Surely, when your staff is reduced to 25 due to this policy, the others will understand it and probably comply.
Tip: don't use the term 'parental control'.
Hi,
I currently work for a small business of around 30 staff and I am looking for a product or software that will allow me to blacklist/whitelist websites.
There are many different types of products available online, but they seem to cost far too much.
One thing to add, all of our agents PC's are working off of the same IP address - not sure if this makes a difference.
Thanks for your help in advance.

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