Re: WAN/LAN IP Address

Tech-Archive recommends: Fix windows errors by optimizing your registry

From: Toby (Nomail_at_ntlworld.com)
Date: 12/09/04 

Date: Thu, 09 Dec 2004 13:04:53 GMT

"AlanNg" <alan2004ng@yahoo.com.sg> wrote in message
news:cp9ccn$a6m$1@newton.pacific.net.sg...
> Hi, recently my office purchase a Cisco 1721 router and the vendor has
> configured the WAN IP as 203.X.X.250/30 and LAN IP as 203.X.X.224/27. The
> WAN IP's subnet mask is 255.255.255.252 in subnet address of 203.X.X.248
> with possible WAN IP as either 249 or 250 which is correct, ie. 250 used.
> As
> for the LAN IP, with the subnet mask of 255.255.255.224, the subnet
> address
> is 203.X.X.224 with possible LAN IP from 225 to 254, but how come the LAN
> IP
> is configured same as the subnet address,ie. 224, isn't there a conflict
> ?!
>
> Next, there are some queries that I don't really understand, hoping
> someone
> can enlighten me;
> - How is subnet address being used, assign to where/what ?!
> - What is the LAN IP for, in the case of router, I thought router should
> only have one IP address ?!
> - Hub is defined in layer 1 or 2 of OSI ?!
> - Can we identify the class of an IP address by the first octet of the
> address, ie. 1-126 must be in class A, 128-191 must be in class B and so
> on, without the need to pay attention to its subnet mask ?!
>
> Please pardon me if I ask silly questions as I'm rather new to these area,
> and hope to rx help asap, thanks !
>
>
I'm not sure how your ISP has achieved this as although theoretically
possible as long as you dont allocate IP addresses on your LAN in the range
203.x.x.248-251 it is not a clever setup, I am not sure of the IOS you have
but if I try this on my 2501 in my lab running I get the following error
messages.

(config-if)#ip address 192.168.0.250 255.255.255.252
192.168.0.248 overlaps with Ethernet0

Ethernet0 is configured as 192.168.0.11 255.255.255.0 but the config allows
the setting but with warning.

(config-if)#no shut
192.168.0.248 overlaps with Ethernet0
Serial0: incorrect IP address assignment

And the Interface stays administratively down.

I would be intrested if anyone else has seen this type of behavour from an
ISP before as if possible could be a problem waiting to happen.

regards

Toby

Relevant Pages

  • Re: Setting up a static IP behind a router
    ... it needs me to enter a default gateway address and subnet mask. ... a router will have the IP address 192.168.0.1 on the LAN side ...
    (uk.comp.os.linux)
  • Re: Router advice needed
    ... Recommendations please for a router that will help me get more IP addresses ... I need more IP addresses on the LAN. ... gateway at 192.168.10.1 and change your DHCP server to hand out new addresses, ... least revised subnet mask. ...
    (comp.dcom.sys.cisco)
  • Re: Router advice needed
    ... subnet is to change the subnet mask on the router interface that has your default ... This means that the router knows about the larger subnet right ... and any host with an inconsistent subnet mask that incorrectly sends LAN-bound ... in and out of the router's LAN interface. ...
    (comp.dcom.sys.cisco)
  • Re: static IP
    ... note that this looks like a dhcp address assigned by your router. ... static ip in your LAN. ... ip, gateway and mask. ... subnet mask 255.255.255.0 ...
    (Debian-User)
  • Re: Network Hardware
    ... LAN With Two Routers" and now wonder whether or not to send this message as ... I was also hoping to use the DLink Router ... have to go through the router to get to the internet on the other subnet. ...
    (microsoft.public.windowsxp.network_web)