Routers and modems can have firewalls builtin. Search your model of hardware in google (in conjunction with the firewall word) to see if there is any.

Ciao

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Liens d'intérêt /Links of interest:
Gnutellaforums en français /The House's rules you have to respect / First search the forum, then create a thread / Free software alternatives! - Logiciels alternatifs gratuits!/