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New Member


Dear All,

I HAVE cisco 1841.

it has 2 interfaces.

the first one which is f 0/0 is have public ip from my ISP.

the other one is normal, and i am going to give it / 24.

now i have 3 subnets totally diffrent .

i want to create 3 subinterfaces from f0/1.

my question is , how many subinterfaces can i add under the f 0/1 ?

and can i make this Router work as NAT ( overloading ) but all the 4 subnet will use the same public IP Address ?

can it be done as per this diagram ?

please update me.

Hall of Fame Super Bronze

Re: NAT-Router


From your diagram, it looks like you have a 3COM switch between the 1841 and the device (Cisco Switch ?).

Any reasons you aren't connecting the 1841 directly to this device avoiding the extra hop on the 3COM switch ?

You can create the subinterfaces in the 1841 as you stated but keep in mind you need to trunk the link at the other end (3COM switch) so it can tag each VLAN you've created.

Based on your diagram, I recommend performing the interVLAN routing at the Core (4507 switch), send the default route over to the 1841 running as single interface -let's say and do the NAT there.

Much simpler configuration.

New Member

Re: NAT-Router

Thanks for your reply.

you said, the 3COM Switch is between 1841 & SW 172.16.0. 6 .

this is correct.

becasue, there is a cable from INT F0/1 to 3 COM sw on port 10.

and from 3 com, there is a cable plugged in in port 7.

I Do not have any VLANS at all.

i am working in FLAT NETWORK.

SO my question is :-

how many sub interface can i add ?

2- how to configure the router with the NAT for Multiple interfaces ?

please update me

Hall of Fame Super Bronze

Re: NAT-Router


You mentioned you don't have any VLANs, but that's what you are creating in the 1841 when configuring subinterfaces. You are terminating multiple LANs into the LAN port (trunk). In order for this to work, both links 3COM and 1841 must be in trunk mode. Your sample config, employing subinterfaces, will take care of the 1841 (make sure to denote the encapsulation dot1q) but you need to manually configure trunking in the 3COM, as well as VLANs.

Your diagram does not show a flat network on the 1841 LAN Port. You have four /24 networks when creating subinterfaces.

If your network is FLAT, what's the reason for creating subinterfaces ?

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