DHCP a bit of a mystery

Ok this bit about DHCP has always mystified me even though I just know theres gonna be a simple explanation - but its one of those things that aint gonna kill me if I dont know the answer - its more like anyone that can give me a neat answer to this will be in my little green book of cool people for a long time.

Ok so I get two routers by the same manufacturer (Cisco) - put them on my network and set them to pick up their public Ip by DHCP. The address is the same for both Routers - now bring on two more routers that are made by a different manufacturer (netgear) now the address picked up by the Cisco's is (lets say) 99.0.0.10 but for both of the netgears its 99.0.43.22.
never changes - I can leave the routers off the network all day and its the same -

Two questions - Why does that happen, and how can I force the netgear to pick up the same IP as the Cisco without resorting to manually entering the ip address ??? Would there be a way of forcing the ISP's DHCP server to return the address I want? - hmm just occurred to me - its gotta be something to do with the routers internal routing tables - hasnt it?

 

yes.


unfortunately, you will proably need to use static IP's and assign all nodes on the network. there are other ways but they include embedded OS's in the routers and idk anything about that.

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another interesting fact is that the Cisco routers are using a different class of network (D) than the Netgear (C), which is wierd. can you post the netmasks of those routers?

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alternatively, i think there is a way for you to assign the secondary routers a static IP address from the primary gateway's. at the same time, have the secondary routers provide DHCP subscriptions to their clients. i hope that makes sense

node (IP in : IP assigning)
gateway (static or DHCP : static)
router (static : DHCP)
client (DHCP)

 

ok, sentient. i learned one way to solve your problem that makes more sense but requiers more hardware.

you will need to change transmission media. lets just use a gateway. you would connect your first router(r1) to the internet connection, then connect a gateway(g1) to r1. now you will be able to connect a second gateway(g2) to g1. then you can connect a second router(r2) to g2.

this way you can connect all devices using DHCP because the routers will not see each other in the OSI model and they will be free of that addressing conflict. the gateway will convert the tranmission to be used on a different media. the routers can use cat5 and the gateways can use fiber. this way there is no confusion and the network stays together.

so...

(internet) --> (r1) <---> (g1) <---> (g2) <---> (r2)

i hope you understand what i am saying here. i will know more about this stuff soon enough. i dont know how to get a print server to work through a gateway yet. and other things.

OR
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simply use a network switch. but it may be more complicated than that. you want to have 2 routers working together.

 

I understand where you are coming from and you did give me a good idea in that second post - but I'll say about that later. I dont know if youre slightly misunderstanding what I am trying to achieve or whether I am slightly misunderstanding your post. Unfortunately I dont have any fibre routers

the problem is simply that if I put a cisco router on my network like this

internet---->cisco box that has the ip address (xample) 100.25.7.99

now I take that box off and replace it with a netgear box and that
will give me a ip address of 100.25.5.34

the problem being how can I force my isp's dhcp server to issue the same address from its servers

I read your second post and I thought it would not solve anything
but then it occurred to me that inadvertantly you may have gioven me the answer. With the netgear box I have it would be impossible as I dont have enough control over its routing tables but the cisco box is a lot better and I do have full controll over that - then it would just be a case of
putting the netgear box on and tracing the route it takes to the dhcp servers and dns servers etc then configuring the internal rtouting of the cisco box with that and any alternative paths found

So thanks for your help Aderall ! youre worth youre weight in gold !
I knew it would be simple just never had my thinking hat on - must be this cough and cold I have caught ! thanks.

can you explain that one just a little more its quite a simple one that .

 

(internet) --> (r1) <--> (g1) <--> (g2) <--> (r2)

i dont know how to get a printer working from a computer connected to (r2) if the printer is connected to (r1). it would have to be connected through the gateways. there is a way to do it but i havent learned it yet. i am learning everything the manual way (vs. automatic). there is probably a program that can make it work but i am not learning the easy stuff right now.

its fun.

 

You can't change your ISP's DHCPD settings. They are most likely assigning DHCP IP addresses computed from the MAC address. Surprising because most ISP's only let you use one MAC address to connect to their service, anything else will lock you out. If you want the same IP I'm willing to bet cloning the mac of the Cisco with the netgear will result in the netgear getting the same IP address that the Cisco normally gets.

 

yes 2cesarwild. that is true that some ISP's using DHCP do assign address with the MAC address. i have never had one but i have heard of it.

 

This is not what is happening tho - I have a cable modem between the routers and the isp that handles the mac address -

I am not asking can I change the isp's dns servers.
What is happening here is that the cisco always gets its ip address allocated from a different dhcp server to the netgear - I just tried the same thing with a sonicwall and that gets a third ip address. Always the same ip address for each router but all different to each other

It must have something to do with the way the router navigates across different networks - I can actually put in my own routing tables on the sonicwall and cisco but not the netgear

all I need to do is ensure they dont pick up dhcp off different servers

 

can you manualy enter the mac address on the cisco. if so, tell the cisco to use the netgear MAC address.

 

what will that work then????
wow - I can actually do that !!!
but are you saying that if I can somehow spoof the netgear mac address this will resolve the prob????

 

there is a good chance that if your ISP see's the netgear MAC address it will assign the Cisco router same IP address as the netgear.

cloning MAC addresses

 

thing is though that both routers clone the mac address of the cable modem and use that. I may actually just talk to the people at the isp to see what they say,

The only reason I want to do this is simply that when I am testing various configs on my network I like to change the routers around especially if I am testing a new one wo I can see how it handles under pressure and other little tests but my network also has a few open ports that require the address to remain constant.

It has been bugging me for ages and one day I will get round to doing something about it