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
0.0.0.0         192.168.224.2   0.0.0.0         UG    0      0        0 eth0
192.168.224.0   0.0.0.0         255.255.255.0   U     1      0        0 eth0

Destination

In the first field, we have a destination IP address of 192.168.224.0, this says that any packet that tries to go to this network, goes out through my Ethernet cable (eth0). If I was 192.168.224.5 and wanted to get to 192.168.224.7, I would just use the network interface eth0 directly.

Notice that we have addresses of 0.0.0.0 this means that no address is specified or it's unknown. So if for example, I wanted to send a packet to IP address 151.123.43.6, our routing table doesn't know where that goes, so it denotes it as 0.0.0.0 and therefore routes our packet to the Gateway.

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.

Genmask

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

Flags

• UG - Network is Up and is a Gateway
• U - Network is Up

Iface

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