I have seen routers and firewalls configured with a public IP address on the internal interface and a private IP address also configured on the internal interface as secondary. Does the internal interface have to connect to a managed switch with Layer 3 capabilities?
I thought it was invalid to share a public and private IPs on the same LAN or interface. Could you point me to...
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a good resource for this information?
Because firewalls are considered quite complex devices and they require a certain level of knowledge and maintenance, most small offices and companies prefer the router and firewall combination, in one box. This is set up once and can then be forgotten in the rack as it happens with most (Cisco) routers! When such configurations are required, the router will be assigned a private IP address on the internal LAN, while the WAN interface obtains its IP from the ISP to which it connects.
In addition, these routers are usually configured with NAT overload mode (see www.firewall.cx/nat-intro.php for more details) so they hide the internal IP addresses from the rest of the world.
Closing, if two logical networks exist and there is one router only, with no firewall between, then the most simple way to connect both of these networks to the Internet would be to assign two IP addresses to the router's Ethernet interface so they router becomes part of both networks.
Dig Deeper on LANs (Local Area Networks)
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