Question DHCP Issue - Some PC not getting internet connectivy

Nov 26, 2018
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Hi Everyone,

I have a bit of a situation here and need your advise on what might be the issue. I have a DC which also host DHCP, now everything was working fine until the DHCP started giving issue (all clients getting 169.254.X.X IP).

After that happened I put in a recommendation to redo the DHCP however we are still waiting on that. But now the issue I am facing is as follows

Their are about 30 PC's connected to the network, some through WIFI AP and some through LAN, yesterday I noticed that their are couple of PC that are getting our internal IP and hence able to access the server but they cannot access the internet. When I try to ping the router from those PC I get no reply. However I have checked the gateway through the DHCP and it is configured with the correct router IP address.

So now my question is, do you guys know what might be causing this? Why do some PC works perfectly and can connect to the internet and others not?

Your help will be greatly appreciated.

Thank you
 

nobspls

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Mar 14, 2018
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DHCP is not giving addresses if your Windows boxes are falling back to 169.254.x.y. IP addresses. You may have devices that are behaving badly and getting new addresses without surrendering the old one first, and so your DHCP server runs out of addresses to give.

You can shorten the lease time to 15 minutes to 1 hour, and this will expire DHCP leases faster so the DHCP server can re-issue the IP addresses sooner. The alternative is to have a bigger pool, or a stricter MAC address to DHCP client IP address mapping so misbehaving devices don't just exhaust your DHCP address pool.
 
Nov 26, 2018
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DHCP is not giving addresses if your Windows boxes are falling back to 169.254.x.y. IP addresses. You may have devices that are behaving badly and getting new addresses without surrendering the old one first, and so your DHCP server runs out of addresses to give.

You can shorten the lease time to 15 minutes to 1 hour, and this will expire DHCP leases faster so the DHCP server can re-issue the IP addresses sooner. The alternative is to have a bigger pool, or a stricter MAC address to DHCP client IP address mapping so misbehaving devices don't just exhaust your DHCP address pool.
Yes I have done that and this part is sorted for now. But the current issue is all clients gets an IP address within our internal IP range but some of them does not get Internet which I have also found out that the cannot ping the main router. This is what baffles me.

Hope you can help, thank you.
 
Nov 26, 2018
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Are they all getting the correct DNS server assigned? What happens if you manually assign this?
If I mannually assign them with an IP and DNS then they get internet connectivity. Something else I just saw now, we have a routing between our site and an external organisation, if I try to ping something from that organisation on the DC then I get connectivity but anything else I get "host not found".

Thank you
 
Nov 26, 2018
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What is the DC? Is that organization got a firewall that is filtering your traffic?
DC is the Domain Controller. I doubt that because I can ping the external organisation from the Domain Controller. I have a suspicion that the issue is with the DHCP also located on the Domain Controller
 
You have something very wrong in your network but it is hard to say what. Because you get ip addresses via dhcp the equipment must have good basic connectivity because DHCP is actually a 2 way communication so the machine can talk to the dhcp server. You should be able to ping the dhcp server.

Why you can not ping the router IP is much harder to say when you have a complex installation. You might have a duplicate IP for the router on the network. See if the ARP entry has the proper mac address of the router on the end stations.
 

nobspls

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Mar 14, 2018
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The domain controller if it does not respond correctly to the DNS request , you will end up with the situation your got now. The domain controller itself may have perfect internet connection, but if is not sharing the DNS information with all the clients.... well you know already.
 

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