How secure is a subnet?

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http://serverfault.com – I have an unfortunate complication in my network - some users/computers are attached to a completely private and firewalled office network that we administer (10.n.n.x/24 intranet), but others are attached to a subnet provided by a third party (129.n.n.x/25) as they need to access the internet via the third party's proxy. I have previously set up a gateway/router to allow the 10.n.n.x/24 network internet access: # Allow established connections, and those !not! coming from the public interface # eth0 = public interface # eth1 = private interface iptables -A INPUT -m state --state ESTABLISHED, (HowTos)