multiple domain's forwarder ip

We have att  T1 line and i added att domain's forwarder ip  my dns server,recently We order comcast dsl line.Some users want to use Comcast DNS. If i add comcast  domain's forwarder ip on my DNS server.Would  T1 user and comcast  be affected?
I disable Comcast modem DHCP,We have server 2003,
KONYALIAsked:
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bcbigbCommented:
I have no clue why some users would want to use Comcast's DNS over anyone else's, unless response time is an issue. Did you order the Comcast DSL to replace the T1? If so, then remove AT&T's DNS entries from your forwarders and replace them with Comcast's. Leaving AT&T's in there could cause those lookups to time out and slow your DNS query process down. If not, and you have two internet connections running at the same time (your T1 and your DSL), then you should pick one set of DNS servers (perhaps a public one, like google's at 8.8.8.8), based on which ones respond the fastest to a ping, and just use those. There are no easy ways to take full advantage of two separate internet connections, however, so I would highly recommend removing one of them if this is the case.

~BC
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KONYALIAuthor Commented:
We are using both of them because some users need to download imaging and if they download imaging with T1 line It makes us very slow.I gave IP as manually and added comcast dns IP,
I wanna be able to do this  without assing manual IP.
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bcbigbCommented:
Ok... the DNS servers don't determine which line you download from. They are merely used to translate DNS names into IP addresses. Once this is done, the traffic is directed by the router and the local machine's routing tables.

The closest way to do this is to have multiple DHCP servers or DHCP server ranges, each of which tells a certain group of users/computers which connection to direct traffic through. One DHCP server will direct users through the Comcast line and one will direct other users through the T1. This will still necessitate manual work to do so, as the DHCP servers have no way to identify the computer except by MAC address. This basically shifts the manual work from doing it at each computer to simply getting the MAC addresses of the computers, deciding which ones go out the Comcast line and which go out the T1 line, and assigning them as such to the appropriate DHCP servers. This option still requires some skill with the DHCP server, and you will need to read up on your DHCP server to make sure they don't conflict with each other. I would suggest the DHCP server built into windows as it's very flexible.

If you want everyone to still communicate locally then everyone gets a local LAN IP address but some of them get the Comcast Gateway IP address (this should be the IP of the Comcast router) and the others will get the IP address of the AT&T T1 router for their gateway. This way they will all be on the same subnet internally, but will access the internet through different routers/gateways/default routes which will direct your internet traffic as you like.

To recap: get a Windows DHCP server set up on your network. Make two DNS scopes with the same subnet (divide it in half, i.e. 192.168.1.1/255.255.255.128 and 192.168.1.129/255.255.255.128). Go to each computer, get the MAC address via an Ipconfig /all at the command prompt. Record these and which router you want that computer to use for its internet traffic. When you're done with this, go assign each one of the MAC addresses to a "static lease" in the DHCP scope that directs that user's internet traffic.

~BC
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bcbigbCommented:
Hi Konyali, I have answered this question completely as far as I can see. Do you have any further questions? If not, please award points and close it.

Regards,
~BC
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