how to change subnet mask

[emily_3]

Hi,
I use the following command to check my linux server. When the destination is default (0.0.0.0), I want to change the Genmask to be 255.255.224.0 instead of 0.0.0.0.
Can you advice me which file I should change? And I was confused that it seems the following three line is configured in different files.

#netstat -rn
Kernel IP routing table
Destination Gateway Genmask Iface
15.104.96.0 0.0.0.0 255.255.224.0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 lo
0.0.0.0 15.104.96.1 0.0.0.0 eth1

[Alexander Chuzhoy]

these lines are your routing table.

To change your subnet mask edit the file
/etc/sysconfig-network-scripts/ifcfg-eth?

where ? can be 0,1 etc

to define permanent route edit the file (it doesn't exist by default
/etc/sysconfig/static-routes

[Stuart Browne]

I also think you're mildly confused as to what a default route is all about!

The default route is where the packet goes if it doesn't know where else to go.

It's the last-resort route.

In order for this to work, it can't be a match for *anything*, thus why it's a 0 bit subnet mask.

If you have a subnet you want to route, that isn't on the local net (15.104.96.0/255.255.224.0), then you need t add a static route (as Alexander has pointed out).

[emily_3]

hi,

But my routing table has two lines for eth1, does that mean I need to add both of them to the same file /etc/sysconf/network-scrips/eth1?

[Alexander Chuzhoy]

the only rules for route you need to add are those that aren't there by default and you wish to define them.
The 0.0.0.0 route may be configured from GATEWAY=x.x.x.x
in
/etc/sysconfig/network
or
/etc/sysconfig/network-scripts/ifcfg-eth1

[emily_3]

Thanks for your advice.
I am trying to add the subnet mask to /etc/sysconfig/network and restarting and waiting for the result.

I would like to know what's the difference between the following directory files:
/etc/sysconfig/network
/etc/sysconfig/networking/
/etc/sysconfig/network-scripts/

[emily_3]

The system boot up.

right now my /etc/sysconfig/network looks like this:
NETWORKING=yes
HOSTNAME=ssrvr260
GATEWAY=15.104.96.1
NETMASK=255.255.224.0
GATEWAYDEV=eth1

But the subnet mask is still 0.0.0.0 for Gateway 15.104.96.1? why?

[Alexander Chuzhoy · Accepted solution]

you don't have to restart the whole system to see the result.
it's enough to restart the network service:
service network restart
/etc/sysconfig/network
is the file with system wide definitions:
like the hostname of the station, the default gateway.
/etc/sysconfig/network-scripts
directory holds the definitions for each network adapter and whatever is assigned with it.
Ip address,type, permission to disable etc


P.S.Please assign points if useful

[Alexander Chuzhoy]

because the file /etc/sysconfig/netowrk holds the line GATEWAY=15.104.96.1
It makes the address 15.104.96.1 as default gateway for 0.0.0.0

[emily_3]

i will assign the rest of the answers finally.

Do you mean that if I use the default gateway the subnetmask must be 0.0.0.0 and can not be changed?
Does the netmask has any problem for this system? since I am trying to use "spdash" comunicate with another server existing in the same subnet, but it failed. while I could ping this server. Don't know why?

[Alexander Chuzhoy]

that's why it's "default" gateway.

to add route to a certain network try this command:
route add -net 10.10.0.0 netmask 255.255.0.0 gw 192.168.0.1

The addresses here are just examples of course.This means to add a route to a subnet 10.10.0.0 with subnet mask 255.255.0.0 via gateway 192.168.0.1

If you can ping some server -that means that you already can communicate with it.
So maybe you trying to access some service on that server that's blocked or unavailable.

[emily_3]

I found the issue due to the lo subnet mask is not correct. Thanks.

Please advice why the outside communication still need the lo setting?

Thanks.

[Alexander Chuzhoy]

Generally it doesn't -what did you change?

[Colin Topliss]

lo is your loopback device. If you get this screwed up, networking will experience problems:

ifcfg-lo has:

DEVICE=lo
IPADDR=127.0.0.1
NETMASK=255.0.0.0
NETWORK=127.0.0.0

You host file should have (if its the default) the following lines:

# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost.localdomain localhost

So, in having a problem with the loopback configuration you would have seen problems with some network services.

Col.