Connect two subnets on Linux

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Problem :

How can two subnets be connected?

I can not ping or traceroute between these subnets.

Vbox1 subnet ifconfig:

eth0      Link encap:Ethernet  HWaddr 08:00:27:f1:68:9e
          inet addr:192.168.1.65  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fef1:689e/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:207703 errors:0 dropped:0 overruns:0 frame:0
          TX packets:41701 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:45480669 (45.4 MB)  TX bytes:4855362 (4.8 MB)

eth1      Link encap:Ethernet  HWaddr 08:00:27:79:02:34
          inet addr:10.0.3.15  Bcast:10.0.3.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fe79:234/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:913 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1335 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:129887 (129.8 KB)  TX bytes:150700 (150.7 KB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:15106 errors:0 dropped:0 overruns:0 frame:0
          TX packets:15106 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:949613 (949.6 KB)  TX bytes:949613 (949.6 KB)

And p4 (because I do not have access to that machine from here, I give a manual-typed summary):

etho : only inet6 addr
lo : inet addr : 127.0.0.1 Mask : 255.0.0.0
venet0 : only inet6 addr
vmbr0 : inet addr : 192.168.0.70 BCast : 192.168.0.255 Mask : 255.255.255.0

Two subnets were connected successful. After switching of ISP I can not access the p4 subnet anymore. So something has changed, but I expect it should be a minor problem, because before using subnets it was working successful.

I have read Virtual machine in two subnets, but the suggestion is to use VLAN. But according to Best way to segment traffic, VLAN or subnet? VLAN works on network layer 2, and subnet on network layer 3. And layer 3, subnets, should be used.

So I should have a gateway between these subnets.

According to Ubuntu KB – Network Config I added to vbox1:

sudo route add default gw 10.10.14.81 eth1

But it generates:

SIOCADDRT: Network is unreachable

Searching returns Ubuntu tunnel or gateway a difference between a host address and a network address.

For your information, vbox1 is Ubuntu 13.04 (Raring Ringtail) and p4 is Debian 7.5 (Wheezy).

I am stuck now.

Solution :

Move vbox1 to a different subnet: Change its IP address on eth0 from 192.168.1.65 to 192.168.0.65 (or some other free IP address in the subnet 192.168.0.0/24). Then, vbox1 knows via which interface p4 (192.168.0.70) is accessible (–> via eth0). vbox1 automatically adjusts its routing table then.
Now you should be able to access p4 from vbox1. If you want to access p4 from any additional client in 192.168.1.0/24, you need to configure vbox1 as a router (i.e. a device that can forward packets arriving on eth1 [192.168.1.0/24] to eth0 [192.168.0.0/24] (and vice-versa). That’s what’s meant by “connecting two networks”).

How to do that depends on the distribution you are using. Basically, you need to write “1” into the device file /proc/sys/net/ipv4/ip_forward.

Like echo 1 > /proc/sys/net/ipv4/ip_forward

This, however is only valid during the current boot cycle. The setting is gone after a reboot.

If you want to do that permanently, you have to put this line into some startup script.
If you are using Ubuntu, you can also put this into /etc/sysctl.conf (I’m not sure about other distros).
Uncomment the line net.ipv4.ip_forward=1.
If you want to have this in effect immediately, you can also issue sudo sysctl -w net.ipv4.ip_forward=1 instead of the ┬┤echo┬┤ mentioned above.

Pls. see also this link on askubuntu.com.

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