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props666999 10-14-2005 10:06 AM

routing tables

This is my route table


Kernel IP routing table
Destination    Gateway        Genmask        Flags Metric Ref    Use Iface  U    0      0        0 eth2  U    0      0        0 eth1      U    0      0      0        eth0      U    0      0        0 lo        84.x.xx.xx        UG    0      0        0 eth0

the is binded to eth2

the ip adreess for that computer is

is the referred to the subnetwork which is binded to eth2

but on eth2 the ifconfig reports ip address

please clarify


Agrouf 10-14-2005 09:27 PM

your ip adress is on eth2 with a netmask of
in binary form, 255 is 11111111 and 0 is 00000000
it means that the first 3 bytes of your ip adress determine the network and the last byte is unique in the network.
the network adress is (it's your ip adress OR'd with your netmask (you only take the bits which are 1 in the netmask)). No machine can have the adress in your subnet because it is reserved for the network adress. Same for which is the broadcast adress (sending to everybody in the subnet).

so when you send a packet to, it will mask it with the netmask -> and route it to eth2
if you send a packet to, it will mask it to -> not -> next route -> (same mask) -> not -> next route -> (netmask -> route to eth0
Now let's send a packet to -> not eth2, not eth1, not eth0, not lo0 -> (default gateway) -> eth0 (again).
It means that your third route is useless, because anything (x.x.x.x) that is not 192.168.0.x, 192.168.1.x or 127.x.x.x will be routed to eth0 anyway (in particular 84.x.x.x)

Note about the netmask : the 1s are always first and the 0s are last (you can not have a netmask of for instance)
We also say that your adress is, because there are 24 bits with a value of 1 in your netmask.

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