Netgear router keeps changing LAN IPs

Zako · 11 May 2011

Hi,

I've got this strange problem with my WGT624v3 Netgear router, as it says in the subject it keeps playing with IP adresses. I have set address reservations for all PCs using this router but they never get the same IP. Everytime I turn my machine on it gets a different IP number and I dont know whats going on. Could you please help me with it?

Thanks in advance

krisdorey · 14 May 2011

So on each machine you have disable dhcp in network settings and given the machine a static ip address (which is outside the dhcp range specified on the router) ?

It's important that you change from dhcp to static ip on each machine no matter what you configure your router for otherwise the pc will just ask for a dhcp lease.

Monkeh · 14 May 2011

krisdorey said:
So on each machine you have disable dhcp in network settings and given the machine a static ip address (which is outside the dhcp range specified on the router) ?

It's important that you change from dhcp to static ip on each machine no matter what you configure your router for otherwise the pc will just ask for a dhcp lease.

A DHCP lease can be static you know.

krisdorey · 14 May 2011

No it can't.

It can have an extremely long lease time but you can't derive a static ip from dhcp, the clue is in the word "Dynamic Host" (unless of course you had a lease pool of 1 ip address which would defy the point of dhcp).

Either that or my Cisco exams have been missing something.

EFCLee1 · 14 May 2011

krisdorey said:
No it can't.

It can have an extremely long lease time but you can't derive a static ip from dhcp, the clue is in the word "Dynamic Host" (unless of course you had a lease pool of 1 ip address which would defy the point of dhcp).

Either that or my Cisco exams have been missing something.

What CISCO exams, this would be covered on 70-640 which is Microsoft

The client is configured to obtain an IP via DHCP, but in Active Directory an IP is statically assigned to the clients MAC address

on-topic, this could be resolved by disabling DHCP on the router and giving all clients a STATIC IP, I have 2 WIFI routers in my home and have done this but not by choice, you cannot have 2 DHCP servers and if you do they will be on different SUB-NETS which will not see each other, if I disabled DHCP on the 2nd WIFI router it still gave all the client attached the wrong SUB-NET so they still could not see the other PC's, answer was to disable DHCP on both WIFI routers and give STATIS IP's to all my clients 8 in total

Hope that helps

krisdorey · 14 May 2011

CCNA, CCNP.

The client is not getting its ip from dhcp then is it? AD might be statically assigning an IP based on the clients ip address but in this case AD is interfering in the DHCP role. I wouldnt even know why you would want AD to gather dhcp leases and then statically assign them as opposed to AD just granting a static ip outside the DHCP range (which is how you should do it). I'm also not sure how many people would have AD configured in a domestic setting unless they are IT. Thus my answer was given for an average joe domestic scenario, which was to assign static ips to each client (as you have also suggested).

Regarding your second point, you could employ ip-helpers to bridge the two subnets. You can of course still have two dhcp servers on the same subnet, it would then just be pot luck which dhcp server responded to the request.

You could also have limited the DHCPOFFER broadcast which would also resolve the issue.

In effect you have created the equivalent of a DMZ, with one range being on one side and the other on the other. So you could also resolve the issue by forwarding packets between the subnets (although I agree this would be very messy as you'd need individual ports for each machine if, for example, you were doing SMB sharing).

Monkeh · 14 May 2011

krisdorey said:
No it can't.

It can have an extremely long lease time but you can't derive a static ip from dhcp, the clue is in the word "Dynamic Host" (unless of course you had a lease pool of 1 ip address which would defy the point of dhcp).

Either that or my Cisco exams have been missing something.

http://www.youtube.com/watch?v=WrjwaqZfjIY

Sorry, been wanting to use that all day.

[code:1]subnet 172.16.0.0 netmask 255.255.255.0 {
pool {
range 172.16.0.100 172.16.0.150;
}
option routers 172.16.0.1;
option domain-name-servers 172.16.0.1;
option ntp-servers 172.16.0.1;
}

host terra {
hardware ethernet 00:50:8d:ba:72:15;
fixed-address 172.16.0.11;
}
host atlantis {
hardware ethernet 00:50:8d:bd:84:b3;
fixed-address 172.16.0.12;
}[/code:1]

Small excerpt. Your Cisco course is missing many things.

Most consumer routers offer this option (Netgear call it 'address reservation', as the OP indicated he's using). There's not a lot you can do about broken firmware unfortunately.

Have you tried rebooting the router?

krisdorey · 14 May 2011

Umm exams not courses

The DHCP range is 100 to 150 in this case, so the reservations are outside the dhcp pool but I'll concede on this bizzare option for a dhcp service to supply a fixed ip outside of the dhcp range.

It still begs the question as to why you would adopt your excerpt?

What advantage does that script router side have over statically assigning the ip client side (especially as its outside the dhcp range anyway. You are also binding the ip to a mac address which could cause two potential issues.

1. MAC spoofing would allow you to gain the ip address, especially if you also employed ARP poisoning and repeated requests.
2. On a server infrastructure , you would be relying on your NIC's MAC address to never change which could well happen with redundant NIC's /failover implementations.

I just don't see the benefit of the router assigning the ip based on MAC as opposed to the client just assuming the static ip.

Monkeh · 14 May 2011

krisdorey said:
Umm exams not courses

Typically a course goes before an exam, and the course is where you get misled. If, however, you've achieved this level of knowledge without formal education, there's only one person to blame..

The DHCP range is 100 to 150 in this case, so the reservations are outside the dhcp pool but I'll concede on this bizzare option for a dhcp service to supply a fixed ip outside of the dhcp range.

It's not bizarre in the least. It's also possible to reserve inside the DHCP pool, it will give you a warning, but function. Although they may have changed that.

It still begs the question as to why you would adopt your excerpt?

What advantage does that script router side have over statically assigning the ip client side (especially as its outside the dhcp range anyway. You are also binding the ip to a mac address which could cause two potential issues.

DHCP is a dynamic configuration protocol, hence the name. I can change any and all settings from one central location, even with the target machine powered off. I can change my NTP server for my entire network in one fell swoop. I can alter the gateway, or push out a new route. All while still maintaining static addresses where required. With the lease time I have set, all this will occur within a matter of minutes, without having to touch more than one machine.

1. MAC spoofing would allow you to gain the ip address, especially if you also employed ARP poisoning and repeated requests.

And I couldn't gain any IP I wanted via those methods anyway?

2. On a server infrastructure , you would be relying on your NIC's MAC address to never change which could well happen with redundant NIC's /failover implementations.

You can specify multiple MAC addresses. It's very easy to get the MAC address for the failover NIC.

I just don't see the benefit of the router assigning the ip based on MAC as opposed to the client just assuming the static ip.

Because you don't need to dynamically configure multiple machines.
I think you are missing the point of DHCP, it is used for far more than merely assigning a random IP address to any old host which comes along.

krisdorey · 14 May 2011

Yes you could get any ip, but the point I was trying to make was that you'd normally see reservations for servers and printers only in a corporate environment, thus the focus on spoofing to get a static ip outside of the dhcp range would be a target.

I've yet to see a corporate infrastructure where they use DHCP services to assign ip's to servers.

Still old dogs learning new tricks I guess, just this old dog might be old fashioned

Out of interest do you employ certification between the dhcp and the host (for important services) ? Otherwise what stops someone planting a dhcp to redirect critical services? With static ip's on the critical service hosts this becomes a lot more difficult to circumvent.

I'm not pretending to know everything, I just have not come across this in any serious infrastructure in the past 20 years.

I think we've hijacked the OP somewhat, and I think we have all given the same advice to assign static ip's as either the OP isn't using the dhcp services you mention for static ip reservation or as you indicated the dhcp service is not performing correctly.

I'll buy you and EFC a virtual beer as a peace pipe

Monkeh · 14 May 2011

krisdorey said:
Out of interest do you employ certification between the dhcp and the host (for important services) ? Otherwise what stops someone planting a dhcp to redirect critical services? With static ip's on the critical service hosts this becomes a lot more difficult to circumvent.

This is my home network. I wouldn't use DHCP for servers, and I would likely undertake certain security precautions in a larger network. Security critical services would not be issued with DHCP, but this would only be a handful of hosts. Static IPs are primarily for convenience anyway, I perform dynamic DNS updates for non-static hosts. All of this goes out the window with IPv6 anyway (can you remember a 128-bit address?).

I think we've hijacked the OP somewhat, and I think we have all given the same advice to assign static ip's as either the OP isn't using the dhcp services you mention for static ip reservation or as you indicated the dhcp service is not performing correctly.

Assuming it exhibits the same behaviour after double checking the settings and rebooting, yes, I'd simply assign the addresses manually on each host.

Well, no, I'd throw out the broken Netgear toy and do things properly, but you can't do that for most clients.

krisdorey · 14 May 2011

Having googled netgear address reservations then it does appear to be known problems, mostly with older firmware/chipset revisions. That or people use the wired ethernet mac instead of the wifi mac for reservation.

So apologies to both you and EFC

EFCLee1 · 14 May 2011

krisdorey said:
Having googled netgear address reservations then it does appear to be known problems, mostly with older firmware/chipset revisions. That or people use the wired ethernet mac instead of the wifi mac for reservation.

So apologies to both you and EFC

No apology needed, it's good to see more "IT Support" literate members on here as we cannot know every detail but collectively be of huge help to the noobies

Zako · 14 May 2011

It's getting quite interesting this topic and I don't get 75% of what you are talking about as I am not IT specialist...

So you are simply saying to disable DHCP?

EFCLee1 · 14 May 2011

Zako said:
It's getting quite interesting this topic and I don't get 75% of what you are talking about as I am not IT specialist...

So you are simply saying to disable DHCP?

Yes, I assume you connect to the router via web browser

Disable DHCP, then note what the IP of the ROUTER is because this will be your DEFAULT GATEWAY, default I think is 192.168.1.1 could be wrong but it will look something like that

Then read this it should help

http://ask-leo.com/how_do_i_configure_my_static_ip_address.html

Netgear router keeps changing LAN IPs

Similar threads