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No Internet from the DHCP

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Saturday, March 7, 2015 2:19 AM

Ok I just set up a brand new 2012 R2 standard server, I have done this many times, the only difference is this is the first time I am using an AT&T fiber circuit.

The problem I am having is I configured the DHCP and I can connect from a computer to the server but none of the connected computers can get internet. The server has internet but not any of the connected computers.

I have checked and check the settings everything seems fine, but I am also looking too hard and may be missing something.

The only difference is I was a little confused by the settings for AT&T and making sure I have the right settings for the NIC card and DHCP.

AT&T provided me with 6 IP's, so I used one of the provided IP's ad the server IP, I used their default gateway and their subnet mask. then in the dns I used my ip as the primary and their default gateway as the secondary.

When I do a test from the computer it tells me "the default gateway is not available".

Any ideas I have tried just about everything I can think of. I am sure it is just something I am overlooking...

All replies (14)

Monday, March 9, 2015 5:52 AM âś…Answered

Hi,

You're welcome :) I wish I was more familiar with ATT's offering here to know exactly what is possible and what isn't.  Based on what you've told me though it sounds like you will need to plug your own routing device into the ATT router so that you can have a private network.

Just plug a routing device such as your Dell PowerrConnect 3548 into the ATT router. Assign the switch an IP address from your 12.220.58.162-166 range and give the switch a default gateway of 12.220.58.161. Now any traffic that gets to the switch will be sent to the ATT router and out to the Internet.

Assign an interface VLAN on the switch to have an IP address on the private network such as 10.1.10.1 and use this address as the default gateway for your private LAN. Plug the Windows Server into the switch and give it an IP address of 10.1.10.2 unless you need this server to have one of the public IP addresses. Configure a scope for 10.1.10.0/24 with exclusions for .1 and .2.  I assume the DHCP server will be your DNS server here based on the information you gave previously, so the scope should have a DNS server assigned that points to your DHCP server, but you could also just use a public DNS server address on the Internet.  Just make sure your Windows DNS/DHCP server has root hints or a forwarder configured and can reach the Internet.

If you want to use the other public IP addresses, just assign them in that range and use 12.220.58.161 as the gateway for them. In this case your DNS server could still be on the 10.1.10 network but you might need a static route here, I'm not sure.

-Greg

P.S. I'm giving this advice off the top of my head so there will likely be some tweaking required. Usually I just fiddle with the configuration of something like this until it's working, using ping and traceroute to check whether or not things are working as expected. It never works right the first time.

Saturday, March 7, 2015 6:56 PM

Hi,

It the DHCP server dual homed? Did you create and activate a DHCP scope and is this what the other computers are using for IP address configuration? If so, what settings did you use in the scope? Is everything connected through a switch or hub?

More information about the network setup is required to diagnose exactly what is wrong.

Thanks,

-Greg

Sunday, March 8, 2015 1:52 PM

I created a scope of 10.1.10.1 - 10.1.10.254 and It is active

right now it is on an unmanaged switch.

I am using one of the AT&T static IP's for my server IP, the router IP as the default gateway, their subnet mask and the DNS is Server IP & Router IP

Now it changed a bit, I was going back and forth with different tests.. once I changed it to this configuration I cannot even get to the DHCP from the client computers. it says "Local Area Connection" Doesn't have valid IP Configuration.

Before when I was getting the DHCP to at least give internal connection, and no internet,  the only difference was the scope was 12.1.12.1 - 12.1.12.254, the rest was the same.

I am just lost... I have done this a ton of times and have never run into this issue, that is why I think I am just overlooking something.

Sunday, March 8, 2015 4:34 PM

Hi,

Check ipconfig/all on a client.

What is the default gateway IP address? Are you providing one via DHCP? Is it in the same subnet as the client (10.1.10.0/24)?  The default gateway must be in this subnet.

What IP addresses are assigned to the DHCP server?

If the DHCP server is in a different subnet from the DHCP clients, then the default gateway must be configured as a DHCP relay.

Thanks,

-Greg

Sunday, March 8, 2015 8:15 PM

Ok that must be what I am overlooking...

The server is using the default gateway and subnet from AT&T, but the DHCP is using a different subnet...

Here is the info, I can't get a ipconfig right now I am not at the office.

The server info:

IP: 12.220.58.162 (static IP from AT&T)

Subnet: 255.255.255.248 (subnet from AT&T)

Default Gateway: 12.220.58.161 (IP for the AT&T Router)

DNS Primary: 12.220.58.162

DNS Secondary: 12.220.58.161

The DCHP Info:

IP Range: 10.1.10.1 - 10.1.10.254

Subnet: 255.0.0.0

I am a little lost because if I change the subnet on the server it does not work and if I change the subnet on the DHCP it does not work (actually it does not allow either one)

 

Sunday, March 8, 2015 8:16 PM

Am I setting this up wrong?

Sunday, March 8, 2015 9:48 PM

Hi,

Yes, this doesn't appear to be configured right.

You have three choices:

1. Don't use DHCP. Configure all the computers with static IP addresses in the 12.220.58 range like you did the DHCP server.  DHCP is not necessarily required, particularly for such a small number of hosts.

2. Use DHCP but configure the scope to provide IP addresses in the 12.220.58 range not the 10.1.10 range.

3. Use DHCP with the 10.1.10 range, but provide a default route in the same IP address range (10.1.10.1 for example) that points to a device that can route your clients off the 10.1.10 subnet. You must have a routing device to do this. If the switch is "layer 3" capable then it can handle this.  A switch that is "layer 3" can do routing. You'll need to check your switch documentation to see if it is capable of routing (and DHCP relay).

The simplest thing to do is #1.

If you want to use DHCP (#2), be sure to assign the subnet mask to be 255.255.255.248 starting at .161, and input .161 and .162 as exclusions. Excluded addresses won't be assigned to clients, which you want here so that .161 and .162 aren't double-assigned by DHCP to a client computer when they are already configured statically on the router and server.

This subnet is also known as a /29 and there are a total of 8 IP addresses in it. If you divide 256 IP addresses into subnets of 8 each starting at 12.220.58.0, the range you are assigned is .160-.167.

The first and the last IP addresses are not usable (they are called the network and broadcast addresses), so that is why you are only able to use 6 addresses (.161-.166). The ATT router is assigned one of those so you actually only get the last 5. You have assigned the DHCP/DNS server to .162 so you must make sure your clients only get the remaining 4 (.163-.166).

If you want to put more computers on the Internet than 4, you'll need to do choice #3 and configure a routing/NAT device a the private IP address range like 10.1.10.x.  A Windows Server can do this if you need it.

I hope this helps,

-Greg

Sunday, March 8, 2015 10:04 PM

Maybe I didn't explain this correctly..I need DHCP I have 60+ computers and about 25 wfi and printers.

I can't use static IP's that Ip range of 12.220.58.161 - 167 are static IP's assigned by AT&T

Right now I was just using a cisco unmanged switch to get things up and running.

I have 2 Dell PowerrConnect 3548 Switches which are layer 3 capable but I have never had to do this sort of setup so I am a little confused by it.

I am just a little confused I have done this many times and never needed any other way, I believe all the servers I have ever set up all had routers (Comcast dsl) that used 10.1.10.1 as their router address

Can you explain to me what I would need to do in order to do #3 this is new to me..

Sunday, March 8, 2015 10:10 PM

I take that back I believe they are only layer 2

Sunday, March 8, 2015 10:47 PM

Here is where I am having to the most trouble understanding.. I have other smaller offices for this company set up with no problem with their own servers using DHCP, but they are all Comcast DSL. So because I have AT&T fiber with their subnet I have to go through all this trouble and equipment just to have the same result?

Monday, March 9, 2015 12:34 AM

Hi,

I see.

Did ATT provide any kind of cheat sheet or configuration guide?  What you really need to know is the private IP address of the ATT router, assuming it has one.  Are you able to connect to this device with a web interface?

Thanks,

-Greg

Monday, March 9, 2015 1:09 AM

No actually it is managed by them.

Yes they gave me a sheet with the default gateway, Static IP list (6), subnet and DNS - that is what I used and the server connect fine but the problem seems to be that I need to have the DHCP on a different subnet in order to get the full range od IP's

Wouldn't the Private IP be the Default Gateway they told me to use, which is the IP of the router?

By the way I truly appreciate all of your help.. I am just going crazy because I have never run into an issue like this.

Monday, March 9, 2015 5:57 PM

I would also recommend that you call ATT and validate what we've discussed here. They might even be able to configure a private address range on the managed router on request. If not, they should at least be able to give some advice for how to ideally use the service given your requirements.

-Greg

Monday, March 9, 2015 6:04 PM

Hi Bslevin,

I would suggest you go out and purchase a firewall/router to place directly behind the ATT Router. Give the WAN IP of the firewall/router the public IP, then place a private IP of 10.1.10.1 on the LAN side.  This will be the default gateway for the network. Place an IP of 10.1.10.x on the server, and implement the DHCP scope, with the server as the DNS server (If it is doing DNS).  This will give you a private IP scheme on the LAN, and the public on the firewall.  Any use of the other IP's that ATT gave you will be configuration on the firewall.  Getting a specific firewall/router is a better option then trying to use one of the Dell L3 switches,  as switches, even layer 3 switches are really not setup to be a gateway device.

Once you have this all in place, every computer will get internet, and you will be setup.