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I'm a former DBA/Developer who now handles all IT aspects in a new role, more by chance than design - you know, "he used to be in IT so obviously he can handle everything" type of thing. I've set up basic home/office networks before, but I do not have an extensive networking background. We're having some issues, and in looking at our router configuration I've found that both a private and public subnet have been set up. The private subnet I get; I'm unsure of why the public subnet would have been set up. From my research so far, it appears that one would only need the public subnet if they were expecting inbound traffic to those machines, which is not a scenario we would encounter. Can anyone give me an idea of why a public subnet would be used? We're just a small office; our main networking needs are internet research and usage of a file server.

Thank you in advance, and I apologize if this seems broad or vague.

This is an ATT/Motorola NVG510 DSL modem/router.

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    Tell us about the make / model of the router you're using. The term "public subnet" sounds like a manufacturer-specific contrivance (and not a standard term in the industry). (The phrasing smells like an old Westell / Frontier DSL modem... I think they're branded NETGEAR now after an acquisition.) Nov 19, 2014 at 19:59
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The purpose of the public subnet functionality is to allow the router to serve devices "outside" its NAT / firewall functionality. (That is, devices with public IP addresses.) This feature is distinct from the idea of forwarding incoming requests from a listening port on the router's public IP to a device "behind" the NAT / firewall (which is handled in the "NAT/Gaming" functionality).

I've used this type of functionality on some devices where the DSL modem is acting only as an edge router and another device is connected to the router and acting as a firewall. It would also be useful to create a "DMZ", albeit that's a little off-putting to me because the device only has a single Ethernet port, so you'd be placing the public and private subnets into the same Ethernet broadcast domain.

If you're not hosting any devices that have public IP addresses assigned to them then you're correct in your supposition that this feature is likely unnecessary.

The manual for the device is mirrored here (which I'm linking because I couldn't find an official Motorola source): http://www.ron-berman.com/wp-content/uploads/2011/11/nvg510manual.pdf

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  • One problem I have seen in the instance of using public IP's instead of NAT on a DSL line is that communication to the servers from the internal network can be limited to whatever speed DSL service you subscribe to (768k for example). That can be highly detrimental to performance.
    – Bert
    Nov 19, 2014 at 20:50
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I'm interpreting "public subnet" to mean public block of IPs.

There's a few reasons for this:

  1. If you order a single static IP address, many ISPs give a /30 block of IPs. This is a block of 4 IPs, 1 of which is usable by the customer (The first IP is the network IP, the last IP is the broadcast, and one of the 2 in the middle would be the router, leaving 1 for your own router.)

  2. If you need more than a single IP address, you may receive a block of 8, 16, etc*. Generally, you need multiple public IPs if you are hosting more than one of the same type of service. As a basic example, if you wanted to have 2 separate web servers, you'd assign public IPs to each of those (well, probably you'd use 1 to 1 NAT through your firewall, but that's a different story.) Other times, you may want to give two physically separate networks access to the Internet. Let's say you wanted to provide an Internet connection for guests. You could buy a second router, assign it one of the public IPs, and have your guests connect to that. It'll be physically separated from your own network, providing reasonably more security.

  3. Someone ordered a block of IPs by mistake and/or they didn't know what they were doing, when you only needed 1 or just a dynamic IP.

*I know Verizon is an exception to this. If you order 5 IP addresses, instead of assigning you a /29 (block of 8), they'll just give you 5 IPs from a /24, using .1 as the gateway.

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