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How to subnet a class C IPp

rayman
rayman asked
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Last Modified: 2008-03-06
I have 1 class C ip address, I got 2 network (40 pc for 1 lan and 20 for another), if I use linux(2 lan card on it) as a router between the 2 lan(not bridge ! because I don't want  too much traffic on both lan), how to config it ?
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Commented:
Lets assume the simple case: you're going to have to subnets with IP-Numbers 1-126 for the first subnet, and 129-254 for the second. Your router will have to IP-adresses. For consistency, it will use the first IP adress in each segment. Lets assume your class C net has IP numbers 192.168.10.0 - 192.168.10.255, and the lan cards on your router are accessible as eth0 and eth1.

Set up the first subnet on eth0:

ifconfig eth0 up netmask 255.255.255.128 broadcast 192.168.10.127 address 192.168.10.1
route add -net 192.168.10.0 netmask 255.255.255.128 dev eth0

Set up the second subnet on eth1:

ifconfig eth1 up netmask 255.255.255.128 broadcast 192.168.10.255 address 192.168.10.129
route add -net 192.168.10.128 netmask 255.255.255.128 dev eth1

The machines in each subnet have to be set up with the correct netmask and broadcast adresses. The router should be the default gateway for the machines in the subnets (use the correct IP number for each subnet).

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Commented:
How to setup my routing table on my internet server(only on lan card on it) ? Where I have two linux, one for connecting to WAN, one for router between 2 lan.

If I get 3 lan, should I use the same configuration as mentioned,
by divided the C class IP into 3 or 4(64 IP each), with netmask 255.255.255.64,255.255.255.128, 255.255.255.192 ?

Commented:
This is getting tricky. Suppose you have the following layout:

LAN1---r1---LAN2---r2---WAN

where LAN1, LAN2 and WAN are networks and r1 and r2 are your routers.

In LAN1 set the machines up for routing in LAN1 and r1 as default gateway.
r1 has to be set up for routing in LAN1 and LAN2, using r2 as default gateway.
Machines in LAN2 have to be set up for routing in LAN2. They need r1 as gateway to LAN1 and r2 as default gateway. This is the tricky part, I don't know if Windows-machines support multiple gateways.
r2 needs to be configured for WAN and LAN2, and it needs r1 as the gateway for LAN1.
If you need details, please increase the points.

Regarding your other question:
You can set up your network as 4 subnets of 64 adresses each, using netmask 255.255.255.192 and network adresses ...0 ...64 ...128 and ...192. You can also use one subnet of 128 adresses and 2 of 64.

Author

Commented:
I've try to add "route add 192.169.10.200 gw 192.168.10.1 dev eth0" on my internet server (r2), where I can ping from any ws on lan2 to the ws on lan1(192.168.10.200), but I don't want to add line by line for each ws on lan1, have you any suggest ?

Commented:
On r2, you can add a routing entry for the complete subnet LAN1:

route add -net 192.169.10.128 netmask 255.255.255.128 gw 192.168.10.1 dev eth0

(I assume LAN1 has adresses ...128-255)

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