Internet · September 28, 2011

Setting up a Static Subnet with PPPoe

I recently changed from a copper wired telephone service and DSL line to a fiber optic connection.

When I lost my DSL, I also lost the five fixed IPs that I was using to host some devices I was using (including webcams)

With my DSL I had five IPs that I could access from the Internet.  But technically speaking I had five IPs on a static subnet my ISP had set up.

It’s not my intent to get into the details of subnetting itself.  Suffice to say that subnets are a way for ISPs to have more IPs for its customers as the number of IP addresses available for use on the Internet disappears.

If you get five IPs from your ISP, odds are you’re using a static subnet (a /29 subnet).

A /29 subnet set up this way results in eight IPs, of which five are useable.  Here’s an example of a /29 subnet an ISP might set up.

The subnet has eight IPs (in this example through

Of these eight IPs, two are unusable; five are useable.  One will act as the gateway IP for your subnet. Unusable Gateway IP of Router Available For Customer Use Available For Customer Use Available For Customer Use Available For Customer Use Available For Customer Use Unusable

The only odd or tricky thing about running on a static subnet is the subnet mask.  For a /29 subnet, the subnet mask will be, instead of the normal  (I’ll get into that again later.)

When I had my DSL, my DSL modem did the routing for my subnet.  All I had to do was plug the various firewall/gateways/devices into the modem and set them up correctly.

But when I switched to fiber optic, I switched to a PPPoe connection and I no longer had a solution for routing for my subnet.

Here’s how I set up my hardware to run PPPoe and a /29 static subnet:

Most importantly is that you need a device that will act as a router and is capable of a PPPoe login.

This part is a little confusing.  A residential gateway is a router.  But normally residential gateways are run in a mode called “gateway”.

You need one that can be set to “router” mode in the firmware.  Note that some residential gateways do not provide the functionality in their firmware to run in router mode (DD-WRT and Tomato replacement firmware both will do this).

This router will log into your PPPoe connection.  It will act as the default gateway for your subnet (yes, the router acting as the gateway runs in router mode instead of gateway mode), and the other devices will connect to it.

I used a WRT54G router with Tomato firmware. The WAN port of the WRT54G is connected to the fiber optic line.

The WRT54G is set up with the PPPoe login and password. Under the Tomato firmware in Advanced Routing | Mode it needs to be set to “Router”.  DHCP server is OFF.

It is assigned the “Gateway IP of the router” as listed in the example above (your IP will be different), with a subnet mask of

Then, the other devices, including other routers and/or webcams simply are simply connected to that router.  The WAN port of each is plugged into a LAN port on the gateway router (in my case the WRT54G).

The other routers are run in gateway mode (the default mode for a residential gateway/router) and are set up just like you would normally set up a home router.  In my case DHCP is on for each.

Each router is set up as a static IP, one each from the list of available IPs for your subnet  For each the gateway IP will be the IP set on the gateway (in my case the WRT54G) router.

However, you use a subnet mask of on each.

Here’s a diagram of how this example static subnet is set up:

PPPoe Static Subnet Routers

With this configuration you will be able to access your routers/webcams using your IPs from elsewhere on the Internet.  Note that the Gateway IP will not be accessible/usable from the Internet though.