How to protect the network from intrusion?

1- firewall,
2- VPN, and
3- security Gateway to Protect the network from intrusion.
And Also.....
Achieve unmatched levels of performance without compromising breadth and depth of security.
Protect business-critical assets—such as networks, servers, endpoints and applications—from malicious threats.
Reduce cost and complexity by consolidating point solutions and integrating with other security tools.

Strong firewall is good..
but if you are in a company or you deal with important DATABASE you have to use VPN

firewall+antivirus and save conection

Use and proper configuration of firewall, which means you have to be careful with the ports you open, use can use proper configuration of switches or create Vlans so that segment of network can only be accessed with different location or department .
Also it depend with network big or small you use VPN in some cases gateway and proper configurations is needed also .
Proper configurations of server on how to deal with intrusion, good user practice on exchange like username and password policy

Use a good fire wall