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Old 17th October 2011
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jggimi jggimi is offline
More noise than signal
 
Join Date: May 2008
Location: USA
Posts: 7,977
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I wrote:
Quote:
If the ALIX box's outer network is a private network, NAT is not required.
I need to add the following clarification. NAT will not be required, but the ISP's equipment will need a routing table entry added in order to be able to route to the inner network.

Here is an example, where the ISP's equipment is presenting a "real" Internet address, and NAT is used to provide a private (RFC 1918) inner network:
Code:
{Internet} a.b.c.d -- [ALIX] -- 192.168.1.1/24 -- [Laptop]
NAT will translate the entire inner network, no matter how big, to a single IP address: a.b.c.d. The ISP's equipment does not need to know anything about the inner network.

But...

Here is an example, where the ISP's equipment is presenting a private network to you -- or, if you decide to continue to use your DSL router you have in place right now, without changing how it is configured, whether ISP provided or not:
Code:
{Internet} [ISP's or your own NAT device] -- 192.168.1/24 -- [ALIX] -- 10.1.1/24 -- [Laptop]
In this case, the ISP's device (or the DSL router) needs to know about the 10.1.1 network. A route must be added to its routing table directing packets to 10.1.1/24 via the 192.168.1.x address of the ALIX router. Without that, the device will route those packets through its default route, which is the wrong direction. They will be routed to your ISP, where they will be dropped.
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