DHCP address range

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gezza2013

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Nov 4, 2013
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Can the DHCP address range go over over the DHCP server's Subnet address range? i.e. dynamic-dhcp range 207.140.96.52 207.141.152.62 when server is server-identifier 207.140.96.49;
subnet 207.140.96.48 netmask 255.255.255.240
Thnak you for your reply.
Gerard
 
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bill001g

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Aug 9, 2012
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It depends if the software would check to insure the interface mask was the same as the dhcp pool. I would suspect you could configure something like this even though it is not real useful. The key part to remember is that the DHCP server does not have to also be the default gateway. Since most home network work this way people make the assumption DHCP server would be the gateway but you could easily have a different gateway so as long as the DHCP server sent out valid subnet masks and gateways to the end clients it would all work. When you use real DHCP servers the server seldom is even on the same lan, they can be located in different cities and are dependent on what is called a helper address. Because of this I suspect the software will let you configure things that are not really valid.
 
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gezza2013

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So, the DHCP server address can be ouside the LAN IP address range. Good. Today, I have another LAN situation: Subnet 12.172.162.64/255.255.255.224. DHCP Svr is 12.172.162.65 - pretty sandard. Then it says: DNS Svr 12.127.16.67 , and Default Gateway: 12.127.206.170 (!). How can that be? Could you please give opinion on the Default Gateway being out of LAN subnet? I just want to make sure that this is an error, before I open my mouth. ;) Thank you so much

 
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bill001g

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It is a error...without doing very strange stuff like putting in a static ARP entry.

The first thing a device does is try to find the mac of the gateway. Some really stupid devices blindly arp whatever address you give them. Of course nothing will answer since it can't be on the subnet. Since it never locates the gateway it can then not send any traffic outside the subnet.
 
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gezza2013

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Guess what bill001g... I got an answer from the guy who wrote the config.. It kinda makes sense, but leaves a few questions unanswered, and I had not seen Ip Default gateway configured this way before. So methinks there is something "extra" working here. Pleae read and let me know what you thin his main switch may have set in it to work like that: "The gateway address is wrong but not for the reasons of wrong LAN. It is not in the wrong subnet. There are 2 subnets in use here. One is the WAN subnet used to interact between CLIENT's demark and us supplying the Main switch. The other is the LAN subnet that is used to interact between our main switch, additional switches and terminals. The default gateway is the port that gives access to outside internet (in other words a port where any switch forwards all packets that it doesn’t know what to do with). Typically for any switch the default gateway will be the internal IP address of a switch / router one level above which it is directly connected to. So, for our main switch this port (IP) has to be in the WAN subnet as that is the closest thing to the wider internet as far as the Metro Cell switches are concerned. In the example below 12.172.162.64 /27 is the LAN subnet and 12.127.206.170 /30 is in the WAN subnet. Now, why is this worng then? Because the correct default IP should be the gateway in the WAN subnet which will be 12.127.206.169 /30. The 12.127.206.170 is the IP assigned to the external interface of our main switch, through which it interacts with the CLIENT's demark. The 12.127.206.169 is the IP assigned to the external interface of the CLIENT's demark network element through which it interacts with the wider internet."..
 
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bill001g

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This is very very non standard. It appears they are over lapping 2 subnets on the same interfaces or they are using proxy arp. I can see how it will work on a outgoing connection. The question would be how they manage to trick the gateway address into ARPing the PC to send the traffic back. They may accept promiscuous arp or something.

This is one of those cases just because you can hack something to make it work does not mean you should. In the long run support is going to be a pain. Most things like proxy arp and promiscuous arp are recommended to be disabled in most routers. This is very similar to the techniques used to do man in the middle attacks and firewalls tend to recognize it and may break a configuration like this.
 
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gezza2013

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I have just read about Proxy ARP, and you could be right. This would only work if all Terminals in the 12.172.162.64 subnet used mask 255.0.0.0, in order to think that 12.127.206.170 is in their own LAN...right? Not quite sure how the promiscuous port works, but maybe the Demarc router, on the way back, has a static route mapped towards 12.0.0.0 or something like that.. Would that work?
 
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