shows that the connection is 10baseT, or common
IP Routing When administering most UNIX-like operating systems, you don’t need to understand routing. The network administrator gives you the IP address of the default route, you put it in the appropriate configuration file, and everything works like magic. [5] OpenBSD systems frequently tend to be part of the network infrastructure, however, or in demilitarized zones where the system must make routing decisions. You really must understand the basics of routing to administer OpenBSD. Routing is simply making a decision on where to send a packet. If a computer is directly attached to a network, it doesn’t need to make any decisions. Your OpenBSD system on the Ethernet network 192.168.1.0/24 already knows how to reach any IP address beginning with 192.168.1; it sends it out that Ethernet. What about an IP address of 209.69.69.12, however? Where should it send those packets? Many computers use a default route, where they send all packets bound for IP addresses that they don’t know about. This is very common in small office networks, where you have one router or firewall that provides network access for everyone in the office. Small companies frequently have only one network, and don’t need complicated routing. The company router itself might have a default route pointing to the Internet service provider, who makes all the actual routing decisions for you. Routed Internal Network Example In a more complicated setting, your system will have to make routing decisions. Suppose your network has multiple routers attached to it, each going to a different network. Machines on your network will have to decide where to send packets. Here’s an example of a fairly common double-firewall situation. This sort of firewall setup is used whenever servers need different stages of protection. The 1 external firewall provides the outermost layer of protection. Any traffic coming in through the Internet hits this firewall first, and any traffic leaving the network goes through this firewall last. This firewall probably has fairly liberal traffic-management rules. The 2 demilitarized zone network is for machines that must be somewhat exposed to the Internet. Perhaps you have intrusion-detection systems here. In many web-farm situations, this is where the actual web servers live. In our example, the DMZ network uses the IP addresses 192.168.0.0/24. The 3 internal firewall is very tightly secured device. Only the bare minimum permitted traffic may pass through it. This firewall is responsible for securing the most vital information on the network. The 4 internal network holds the most vital, protected information on the network: financial information, customer databases, or your MP3 collection. In our example, the internal network has the IP addresses 172.16.0.0/24. Many of the hosts in this network have very simple routing decisions. Anything in the internal network has only one route to reach anything. If the packet is going to an IP address not in the 192.168.1.1/24 network, it must be sent to the 7 default gateway on the internal network. Similarly, the internal firewall has two networks directly attached. If it wants to send a packet to an IP within 172.16.0.0/24, it sends the packet out the 7 interface directly attached to that network. If it wants to send a packet to an address within the 192.168.0.0/24 range, it sends it to the 6 interface directly attached to that network. If it wants to reach an IP that isn’t in those two ranges, it uses the default gateway of 5 192.168.0.1. The external firewall is directly attached to the 192.168.0.0/24 network, so it can send packets there. It’s directly attached to the Internet and can send any packets it doesn’t know how to reach otherwise out there. That leaves out the 172.16.0.0/24 network, however. Packets bound for 172.16.0.0/24 should be Page 174
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