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Home > Archive > CCNP > November 2001 > IP address = Gateway
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IP address = Gateway
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| Certaddict 2001-11-28, 8:43 am |
| Hi,
Can somebody enlighten me as to why IP address = gateway and why subnet mask = 255.255.255.255???
PPP adapter GlobeSpan Dial-Up PPP Connection:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 210.24.215.121
Subnet Mask . . . . . . . . . . . : 255.255.255.255
Default Gateway . . . . . . . . . : 210.24.215.121
Above is the ipconfig result from a local ISP. | |
| strikeattack 2001-11-28, 1:55 pm |
| It looks to me like they are doing some very special configurations here. According to the configuration, any packets originating from the device will be considered remote and will be routed through itself. The node is functioning as a router and because of the IP address and subnet mask, any address other than itself will be remote, so all packets originating will be routed. | |
| The Reamer 2001-11-28, 2:49 pm |
| The other reason why this is done is to force the machine to send out arp requests for any other node. There could be some timing issues that are involved that would call for a situation like this.
Reamer | |
| Certaddict 2001-11-28, 6:25 pm |
| I called up the ISP and was told by one of the helpdesk guy that this configuration is to help enhancing security for individual node that dials up.
Anyway, I am still utterly baffled and have not come across this in any books so far.. | |
| sidodgers 2001-11-29, 5:48 pm |
| quote:
Originally posted by Certaddict
I called up the ISP and was told by one of the helpdesk guy that this configuration is to help enhancing security for individual node that dials up.
Anyway, I am still utterly baffled and have not come across this in any books so far..
No, no, no  (well, at least, experience contradicts this...)
the reason that there's a /32 mask is because it's a point-to-point connection. Ordinarily, the default gateway would be set to the machine at the other end of it (and since it's a p-t-p conn, none of the normal subnetting rules apply and the machine at the other end could have any legal IP address it wanted to.)
Either way, the configuration means the same thing, all that the 'default gateway=my IP' means is that it's going to send all traffic for which there isn't a better route (probably all of it unless you've got a network card in that machine as well) over the PPP link. It's the same as setting a default route through an interface on a Cisco.
Cheers,
Sid | |
| strikeattack 2001-11-30, 6:46 pm |
| quote:
the reason that there's a /32 mask is because it's a point-to-point connection.
With a 32-bit mask, the configuration would not be specific to a point-to-point circuit. All this means is that all addresses other than its own are on remote subnets and it will have to route its own packets to reach them.
quote:
(probably all of it unless you've got a network card in that machine as well
It will ROUTE ALL traffic. There is no question about it. It is has no choice because it is compelled to do so by the rules of IP. With a 32-bit mask, the network ID is to large to accomodate any more hosts in its defined subnet.
Certaddict, on whatever node this is, are you running any dynamic routing protocols, IP ROUTE statements, static routes, or anything that would tell the machine what the next available hop address is? You would have to in order to speak outside of your box.
This is all assuming of course that the machine is not dual-homed.
I await any differing opinions, if anyone has any. | |
| dmaftei 2001-11-30, 7:36 pm |
| Here's what the good ol' folks at Cisco Systems, Inc. say about the issue:
quote:
Originally posted at www.cisco.com
No IPCP Options for Default Gateways and Subnet Masks
The NAS and the Windows PC establish a point-to-point connection that runs PPP. The PC acts as a host that does not route IP traffic between multiple interfaces. The PC automatically uses the IP address of the NAS (learned during IPCP negotiation) as the default gateway. The subnet mask is not needed in that point-to-point environment. The PC knows that if the destination address does not match the local address, the packet should be forwarded to the default gateway (NAS) which is always reached via the PPP link.
Inaccurate Information in Winipcfg/Ipconfig
Microsoft opted for displaying the address assigned to the PC as the default gateway address and showing the classful mask for that address instead of leaving those fields blank. Don't worry about this information if IP connectivity through the dial-up adapter is operating correctly.
And now, to end the suspense... here's --> THE LINK <--  | |
| strikeattack 2001-11-30, 9:25 pm |
| dmaftei, no personal comments about the setup outside of the Cisco link? Usually you have a comment or two rather than just a Cisco quote... | |
| dmaftei 2001-11-30, 10:10 pm |
| quote:
Originally posted by strikeattack
Usually you have a comment or two rather than just a Cisco quote...
First of all dial-up is one of my weakest points  Then, I've been quite busy lately, and so I didn't have too much time to 'chat' with you, guys... Maybe later... |
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