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New Member

Public Ip

Hi all,

i am having cisco 2520 router and has 64 kbps link to internet which is using thru NAT. We have got 6 no public ip from our ISP and we r using only 1 for NAt , so if i want to use any of public IP to my PC in LAN how should i configure ....pls help me i am verrry much confuse...

shall i remove NAT to use that..????

thanks in advance..

regds,

Mangesh

1 ACCEPTED SOLUTION

Accepted Solutions
New Member

Re: Public Ip

Hi Mangesh,

You are making a simple problem look worse. what the problem is you are trying to assign eth0 of your router the IP address from the same subnet as your Serial interface.That won't work,b'cos each interface of a router is in different broadcast domain.What you want to do is you can ask your ISP to give you a seperate range of IP's ( different subnet to give to your LAN ) and you don't need 6 IP's just to use for a WAN link.So ask the ISP to give 2 IP's ( 255.255.255.252 subnet) for your WAN interface and the rest required by you for your LAN.

And of the IP's given for your LAN give one IP from that as secondary to your eth0 ,and assign the PC's in your network IP's from the same subnet.Let the NAT remain like that ,it should work.

Regards,

Homin

7 REPLIES
New Member

Re: Public Ip

Mangesh,

You should be able to statically map any of the ip addresses that were assigned to you from your ISP. You may want to check out http://www.cisco.com/en/US/tech/tk648/tk361/technologies_q_and_a_item09186a00800e523b.shtml for some general answers on NAT. If you need another public IP beyond the ones you have been assigned, you will have to contact you ISP for help.

Bob

New Member

Re: Public Ip

thanks bob for ur prompt reply,

now i will tell u my exact scenario...

I am having 6 public ips from ISp of which one i assigned to S3 and now my intranet is having subnet 192.168.11.0 so obiviously my E0 address is 192.168.11.1

Now problem is, i want to assign any of my PC in my intranet the Public Ip out of 6 , but when i am trying it , it only pings itself not from anyother PC.

I tried with 2 lan cards on NT and assigning one private IP and other public , givving diff. combinations of gateways....but no luck

Even i removed NAT today to checkup my luck :(

I will be verry relax if u can help on it..

thanks ,

mangesh

Re: Public Ip

Hi, IF you assign a public ip to one of the PCs, you also would need an ip address assigned to the router from the public IP pool.

What you can do is, assign one of the public address say 200.200.200.1 for NAT, 200.200.200.2 for router as secondary ip address, and 200.200.200.3 through 6 you can use for your PCs.

Now, once you configure this, your router ethernet would have two directly connected networks, one for 192.168.1.x and other for 200.200.200.x networks.

Pinging between PCs with public IP and private IP would work, because router will route between the PCs. On the PCs with public IPs, configure 200.200.200.2 as default gateway.

New Member

Re: Public Ip

hi,

thanks for ur reply..

but when i am trying to put the public ip to Ethernet port of my router it gives me error that "200.200.200.0 is already assigned to Serial 3" as i have assign one of my public ip to Serial 3 to which internet link connected.

suppose i am having ips from 200.200.200.8 to 13 and subnet mask is 255.255.255.0

i gave ser 3 ip 200.200.200.8 255.255.255.240

and now if i try to give Eth 0 ip 200.200.200.9 255.255.255.240

it gives me error "Subnet 200.200.200.0 is already used by Ser 3"

If i am givving Ip to Eth 0 first and then to Ser 3 then again error comes but it comes now from Eth 0.

pls help me...

thanks mangesh

Re: Public Ip

HI,

I understand now that you are using PAT with the IP address of the serial interface of your wan router.

You cannot assign an ip address from same subnet to two interfaces of a router. As somebody previously stated, you really dont need 6 ip addresses for communicating to internet. But you can surely ask your ISP to give you different block of addresses, one for your wan (/30 mask) and one for your lan.

What you can also do with addresses from 12 to 13 is, this. In this procedure, you might not be able to use .8 and .12 ip addresses. What i am suggesting is a subnetting of your ip addressing scheme.

200.200.200.8/30 and 200.200.200.12/30

.8 is now invalid as its a network address. You cannot assign that ip to any interface.

.9 .10 are valid. .11 is broadcast address for the .8 subnet.

.12 is next subnet with .13 as the first ip address.

Since you do subnetting, now you have only 3 valid ip addresses, but falling into two subnets, viz, 200.200.200.8/30 and 200.200.200.12/30.

You can assign 200.200.200.13/30 ip address to your wan interface and the rest 2 ip address can be used in your lan. One on your router as secondary ip addresss, and the other on a PC itself.

But you lose a lot of ip addresses, by doing this way. The better would be to ask the ISP to give you two blocks of ip addresses, which is not really needed unless you plan to use some public servers to be accessed from the internet. Even that can be handled easily with outside to inside NAT.

New Member

Re: Public Ip

Hi Mangesh,

You are making a simple problem look worse. what the problem is you are trying to assign eth0 of your router the IP address from the same subnet as your Serial interface.That won't work,b'cos each interface of a router is in different broadcast domain.What you want to do is you can ask your ISP to give you a seperate range of IP's ( different subnet to give to your LAN ) and you don't need 6 IP's just to use for a WAN link.So ask the ISP to give 2 IP's ( 255.255.255.252 subnet) for your WAN interface and the rest required by you for your LAN.

And of the IP's given for your LAN give one IP from that as secondary to your eth0 ,and assign the PC's in your network IP's from the same subnet.Let the NAT remain like that ,it should work.

Regards,

Homin

New Member

Re: Public Ip

Hi Homin,

Thanks a ton dear..

its really matter of basic concepts in networking in which i was lagging but now u guys has really guided me nicely so thanks a ton again...

mangesh

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