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Community Member

NAT QUESTION - PLEASE HELP

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 192.168.1.100 / 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.

4 REPLIES
Community Member

Re: NAT QUESTION - PLEASE HELP

issue the "show idb" command to see how many subinterfaces your router supports. Your diagram certainly looks doable. I don't know if your router can actually handle NATing for 1000 IPs.

A couple points on your diagram. It looks like you have four subnets. You need to move your first network off of the physical interface to a sub-interface. You really shouldn't be using public IP space on your inside network. Does your diagram show your old and new network?

Community Member

Re: NAT QUESTION - PLEASE HELP

THanks for your reply.

i have the followig results.

=============================================

HO-RO-Internet#sh idb

Maximum number of Software IDBs 1200. In use 12.

HWIDBs SWIDBs

Active 6 6

Inactive 6 6

Total IDBs 12 12

Size each (bytes) 2904 1280

Total bytes 34848 15360

Type SIdx Idx St,O,Sh Interface Name (subblocks)

---------------------------------------------

H 1 1 U,D,R FastEthernet0/0 (HW SB CDP(4), MAC ADDR(2), Ether(1))

H 2 2 U,D,R FastEthernet0/1 (HW SB CDP(4), MAC ADDR(2), Ether(1))

H 3 3 A,D,R Serial0/0/0 (HW SB CDP(4), Serial(3))

H 4 6 U,D,R Loopback0

H 5 7 U,D,R Loopback1

H 6 8 U,D,R Loopback3

S 1 3 U FastEthernet0/0 (SW CDP(5), DSS(4), Dynamic DNS Updates(3

), NetBIOS(2), KEEPALIVE(1))

S 2 4 U FastEthernet0/1 (SW CDP(5), DSS(4), Dynamic DNS Updates(3

), NetBIOS(2), KEEPALIVE(1))

S 3 5 U Serial0/0/0 (SW CDP(5), NetBIOS(2), KEEPALIVE(1))

S 4 9 U Loopback0 (KEEPALIVE(1))

S 5 10 U Loopback1 (KEEPALIVE(1))

S 6 11 U Loopback3 (KEEPALIVE(1))

Key: SIdx=Sort Index, Idx=hw_if_index or if_number

St=Current State, O=Old State, Sh=Shadow State

A=Admindown, D=Down, G=Going Down, I=Init

R=Reset, T=Testing, U=Up, X=Deleted

HO-RO-Internet#

===========================================

so, from where can i know how many sub-interfaces i have ?

please update me .

Community Member

Re: NAT QUESTION - PLEASE HELP

Your question changed from "can add" to "I have."

From your output, you can have 1200 total interfaces. You will likely run into memory issues before you hit 1200 interfaces.

Blue

Re: NAT QUESTION - PLEASE HELP

YES,

it can be done. a few notes though.

your F0/0 interface will not have an ip address on it. (only the f0/0.1, f0/0.2..etc)

if you use subinterfaces, you only put ip addresses on those.

(likewise, if you do not use subinterfaces, you would put an ip address on F0/0)

you can then enable NAT to provide internet acceess for all of your subnets. you will create a nat pool that all subnets are assigned too and then use an 'ip nat inside source list....overload' command. see the link below for an more NAT info:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800ca6b4.html#wp1002272

also, you can configure a subinterface for each VLAN you need to configure.

(since your interface will be supporting multiple vlans, it will be using 802.1q and the switch port it connects to will have to be configured as a 802.1q trunk as well)

you can easily tell how many subinterfaces you have by either 'show run' and count them up, or 'show interfaces' and count them.

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