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I am a relative newcomer to networking (and my current employer), and want to understand whether our present set up is okay, or not.

We have 8 static IPs provided by our ISP. We have a Windows SBS 2003 server with 2 NICs - one on the public side, with one of our static IP addresses; and one on the LAN with a number in the 192.168.100.nnn range. This all seems normal for SBS 2003.

The SBS is used as our gateway and has an ISA firewall running on it. The router/modem, on the public side of the SBS, shows its LAN IP as being another one of our static public IP addresses. On the WAN side of the router, the IP is, as expected, a public number, but not within our range. I can only assume that this was also provided by our ISP, and will be asking them as such.

With the above in mind:

Is the use of a public IP on the LAN side of the router because the router is effectively outside of our LAN?

If not, is this use of a public IP address on the LAN side of the router okay?

Thanks in advance.

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up vote 4 down vote accepted

Yes, that is normal. The WAN side of your router will have IP addressing that your ISP uses in it's own network - probably a just a /30. The LAN side of your router will have your public address range on it. Your ISP will then route your public address range at the WAN IP of your router, allowing you to use your own public address space on the LAN side of your router.

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+1 Only person to actually read the whole question so far... Could possibly throw "DMZ" or "Perimeter Network" into the mix if you want to get all technical on him. =] – Chris S Dec 29 '11 at 14:47
I agree with @Chris - thanks for reading the question properly. You have reassured me that this is normal. The description of the public address space between the router and the SBS makes sense. Thanks – iWeasel Dec 29 '11 at 14:59

If you are saying that you have a public (WAN Interface) and private (LAN interface) address on a router, then the setup is correct.

For example

  • Server -
  • DHCP setup on server a private IP assigned (such as for the router
  • DHCP - scope for private ip addresses that can be assigned to client
  • Router - this has a public address such as whilst private would be
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Public IP addresses are globally visible. Your ISP provided you with a set of public IP addresses which should be behind their end modem (because their routing tables). Any device that you want to be publicly visible should be in that LAN, if you don't use some kind of inbound NAT.

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