2. Routing Table

Look at your machine's routing table:

pete@icebox:~$ sudo route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface UG 0 0 0 eth0 U 1 0 0 eth0


In the first field, we have a destination IP address of, this says that any packet that tries to go to this network, goes out through my Ethernet cable (eth0). If I was and wanted to get to, I would just use the network interface eth0 directly.

Notice that we have addresses of this means that no address is specified or it's unknown. So if for example, I wanted to send a packet to IP address, our routing table doesn't know where that goes, so it denotes it as and therefore routes our packet to the Gateway.


If we are sending a packet that is not on the same network, it will be sent to this Gateway address. Which is aptly named as being a Gateway to another network.


This is the subnet mask, used to figure out what IP addresses match what destination.


  • UG - Network is Up and is a Gateway

  • U - Network is Up


This is the interface that our packet will be going out of, eth0 usually stands for the first Ethernet device on your system.


Look at your routing table and see where your packets can go.


Where are packets routed to if our routing table doesn't know?