I have a question hopefully someone may be able to provide an answer for. My building has 3 floors. Each floor has it's own subnet (10.10.x.x for first, 10.20.x.x for second, 10.30.x.x for third). Each floor has it's own gateway. They are 10.10.0.1, 10.20.0.1, 10.30.0.1 and the subnet mask is 25.255.0.0. When people with laptops move between floors they retain their original IP rather than pulling a new scope and as a result are unable to communicate with anything on the network or even open Internet Explorer. The same thing happens when relocating a desktop from one floor to the other. Any suggestions would be helpfull.
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IP ADDRESS QUESTION 3
- Thread starter pll7538
- Start date
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1
- #2
LawnBoy
MIS
- Mar 12, 2003
- 2,881
You could set every subnet mask to 255.0.0.0 and it would fix your problem, but it would also defeat the purpose of isolating the networks to begin with...
This is what you need to fix. Each floor has a completely isolated address range. The client PCs will have to match the subnet they are connecting to, either by manually changing their address or by DHCP.
If you are using Windows XP or 2000, you can have them open a cmd window and type ipconfig /renew. That way they will retire their current IP address and request a new one.
Shouldn't be that big of a deal for occasionally moving between floors.
pansophic
Shouldn't be that big of a deal for occasionally moving between floors.
pansophic
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2
- #4
Where is the DHCP server located? (one of the 3 subnets ore some other one with all the servers for instance)
What are the parameters of the 3 scopes you defined on the DHCP server?
How are IP helper addresses setup up on your 3 routers?
G.
What are the parameters of the 3 scopes you defined on the DHCP server?
How are IP helper addresses setup up on your 3 routers?
G.
- Thread starter
- #5
What DHCP server is this (Windows 2003?)
Is this 192.168.1.x a separate subnet that is used for something else? How is it connected to your 3 10.x.x.x subnets?
What routers are you using?
A lot of questions, not many answers ;-) (yet)
G.
Is this 192.168.1.x a separate subnet that is used for something else? How is it connected to your 3 10.x.x.x subnets?
What routers are you using?
A lot of questions, not many answers ;-) (yet)
G.
How is your DHCP server that is on a 192.168.1.x address identifying where the request is coming from? From your original description, there are only a couple ways that I could see DHCP working. One would be if your router (which is providing routing for the different subnets) is your DHCP server. It should identify where the request is coming from based on the interface that the request arrives at and provide a network specific address. The only other alternative that I could see would be having a separate DHCP server on each subnet.
Has this ever worked?
pansophic
Has this ever worked?
pansophic
There is another possibility and that's the use of IP helper addresses (hence my question regarding this in a previous post)
The IP helper (as it is called on Cisco routers) forwards the broadcast sent by the client to the DHCP server (using a unicast). It also inserts its own address in the gateway (GIAddr) field of the request (not to be confused with the router option of a scope). When the DHCP server receives such a request (i.e. with the GIAddr field non-zero) it will respond with an address from a scope corresponding to the IP of the gateway.
This is the setup that I would use in pll7538's environment, 1 DHCP server with 3 scopes (one for every subnet) and IP helpers on the Router interfaces. If your router does not support this you can install DHCP relay agents on a PC(?) or server in the subnet which does the same job.
If there are already 3 DHCP servers used I think there would not be a problem. Every DHCP server serves addresses in its own subnet. A laptop moving from floor 1 to 2, would do a DHCP request in order to check if it can continue to use its lease (received from DHCP server of floor 1). The server on floor 2 would need to answer with DHCPNAK as it is not serving IP's in this range. Subsequently the client would start a new request without its address of floor 1 and receive a correct address from the server on floor 2. This is the same thing as when you take your laptop home after work. When booting it will first ask if it can continue to use the IP from the office range. Your router at home will NAK this, the client starts all over from scratch and you end up with an IP in your home range.
Some interesting stuff about DHCP can be found at:
DHCP Finite state machine:
Fields used in the DHCP messages:
Geert
The IP helper (as it is called on Cisco routers) forwards the broadcast sent by the client to the DHCP server (using a unicast). It also inserts its own address in the gateway (GIAddr) field of the request (not to be confused with the router option of a scope). When the DHCP server receives such a request (i.e. with the GIAddr field non-zero) it will respond with an address from a scope corresponding to the IP of the gateway.
This is the setup that I would use in pll7538's environment, 1 DHCP server with 3 scopes (one for every subnet) and IP helpers on the Router interfaces. If your router does not support this you can install DHCP relay agents on a PC(?) or server in the subnet which does the same job.
If there are already 3 DHCP servers used I think there would not be a problem. Every DHCP server serves addresses in its own subnet. A laptop moving from floor 1 to 2, would do a DHCP request in order to check if it can continue to use its lease (received from DHCP server of floor 1). The server on floor 2 would need to answer with DHCPNAK as it is not serving IP's in this range. Subsequently the client would start a new request without its address of floor 1 and receive a correct address from the server on floor 2. This is the same thing as when you take your laptop home after work. When booting it will first ask if it can continue to use the IP from the office range. Your router at home will NAK this, the client starts all over from scratch and you end up with an IP in your home range.
Some interesting stuff about DHCP can be found at:
DHCP Finite state machine:
Fields used in the DHCP messages:
Geert
- Thread starter
- #9
Thanks for all your input. I don't have many more answers on this end as this whole network was configured by a third party and the guy who is in charge of it does not know much about Cisco routers. The sole DHCP server is on it's own subnet. My guess is that the helper addressing would be the solution. I have no idea why they would have configured a separate gaetway address for each different subnet but it boggles my mind that when a person takes a laptop from their desk on the 3rd floor and plugs it into a jack in a meeting room on the second floor that they stay with their original ip and do not get assigned a new scope. Anyhow, tomorrow is my last day here so I am not going to worry about it too much and just let this network guy boggle his way around. I have never seen a network so messed up in my life.
Interesting tidbit gdvissch. I haven't used a helper address in at least 10 years. I recall that we used it for an application that did broadcasts to update the clients and we needed clients on the entire network updated, not just the local subnet.
This problem seems very strange in that any scenario that would assign addresses correctly by subnet, should reassign them correctly. Sounds like we will never know why though.
pansophic
This problem seems very strange in that any scenario that would assign addresses correctly by subnet, should reassign them correctly. Sounds like we will never know why though.
pansophic
VLAN or physical segment is very similar if not the same.
The only thing I can think of is that there is some problem with subnet masks on routers, the PC's, the scopes ...
A network trace would probably give some more insight, but I guess we'll never know :-(
CU all
G.
The only thing I can think of is that there is some problem with subnet masks on routers, the PC's, the scopes ...
A network trace would probably give some more insight, but I guess we'll never know :-(
CU all
G.
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