External networks must be carefully considered as part of the overall security strategy of the local network. A router may include a firewall, VPN handling, and other security functions, or these may be handled by separate devices. Many companies produced security-oriented routers, including Cisco PIX series, Cisco Meraki MX series and Juniper NetScreen. Routers also commonly perform network address translation (which allows multiple devices on a network to share a single public IP address) and stateful packet inspection. Some experts argue that open source routers are more secure and reliable than closed sourcerouters because open source routers allow mistakes to be quickly found and corrected.