Default Gateway Outside of subnet

[jbarbara11]

I ran into a IP config last week that I haven't seen before. A friend of mine has a network/subnet like this: 205.88.99.33 subnet 255.255.255.224 but the default gateway is 209.161.144.237 (IP address are made up for this posting, I don't know whose addresses these are).

I would think that the Default Gateway would have to be on the same subnet (something between 205.88.99.33 through 62). Does it have to be or can the default router be anything valid as long as it is the edge router?

Thanks,
jbarbara11

 

[hnd]

Usually I think you are right, but i have learnt: In networking some things are working even if there is nothing logical.

In each case it is a strange configuration and I am sure, trouble will occur at least if something is not working properly. Happy Troubleshooting.
hnd
hasso55@yahoo.com

 

[dxd]

It would be my guess that the default gateway address is invalid. However, we should think of the default gateway as " the route of last resort" meaning that if the computer knows how to get somewhere through other routes defined in the routing table, then it does not have a use the default gateway and as such you will not have a problem.

So, check out the route table "route print" and see if other routes are defined.

I may be wrong, but that is the way I remember it.


Doug
dxd_2000@yahoo.com

 

[Guest_imported]

This will only work if the local router has learned a route to the distant router that was defined as the “default gateway” on the workstation. In this case the local router is providing proxy ARPs because it knows of the distant network that the “default gateway” is on. Check-out the following excerpt:

(The following note is from a Microsoft troubleshooting procedure

"Verify Default Gateway
Next, look at the default gateway. The gateway address must be on the same network as the local host; if not, no messages from the host computer can be forwarded to any location outside the local network. Next, check to make sure that the default gateway address is correct as entered. Finally, check to see that the default gateway is a router, not just a host, and that it is enabled to forward IP datagrams."


What is Proxy ARP?

On a network that supports ARP, when host A (the source) broadcasts an ARP request for the network address corresponding to the IP address of host B (the target), host B will recognize the IP address as its own and will send a point-to-point ARP reply. Host A keeps the IP-to-network-address mapping found in the reply in a local cache and uses it for later communication with host B. If hosts A and B are on different physical networks, host B will not receive the ARP broadcast request from host A and cannot respond to it. However, if the physical network of host A is connected by a gateway to the physical network of host B, the gateway will see the ARP request from host A. Assuming that subnet numbers are made to correspond to physical networks, the gateway can also tell that the request is for a host that is on a different physical network from the requesting host. The gateway can then respond for host B, saying that the network address for host B is that of the gateway itself. Host A will see this reply, cache it, and send future IP packets for host B to the gateway. The gateway will forward such packets to host B by the usual IP routing mechanisms. The gateway is acting as an agent for host B, which is why this technique is called "Proxy ARP"; we will refer to this as a transparent subnet gateway or ARP subnet gateway. When host B replies to traffic from host A, the same algorithm happens in reverse: the gateway connected to the network of host B answers the request for the network address of host A, and host B then sends IP packets for host A to gateway. The physical networks of host A and B need not be connected to the same gateway. All that is necessary is that the networks be reachable from the gateway. With this approach, all ARP subnet handling is done in the ARP subnet gateways. No changes to the normal ARP protocol or routing need to be made to the source and target hosts. From the host point of view, there are no subnets, and their physical networks are simply one big IP network. If a host has an implementation of subnets, its network masks must be set to cover only the IP network number, excluding the subnet bits, for the system to work properly.

Dave

 

[Guest_imported]

I should have mentioned that the Microsoft note indicates that your friend’s workstation is misconfigured, and I totally agree. This is not a valid configuration.

Dave